There's a lot of confusion on this question.
Here are two totally different paradigms of discussion about "shares and percentages"
(1) Go to angel.co
(By the way OP, your first stop should be angel.co to review the market.)
So regarding startups that are already "underway" ...
Note that, a VC will think of and describe these are "very first days" startups. But TBC they are already established on paper, have an office, all the founders, all paperwork done, logos and such crap, and likely
You can should and must go browse angel.co to come up with such a percentage. the fact is you'll end up with a (meaningless) figure like "2.5%".
All of that is for what we would call "established" startups and what VCs think of as "absolutely just got going."
(2) Item 2 is totally different. It's when 2 or 3 or 4 people sit around and say "let's start an ecommerce site, I have this great idea that it should only sell yellow clothes, yelloware.com"
As I understand it, this is what the OP is talking about.
Here are actual examples:
i) Project B. A guy and a money guy. Money guy will donate a bag of cash with $150,000. Money guy will get 30% - 60% (just a negotiation). They considered adding an engineer (who'd want 5-25%, just a negotiation), a well-known designer bloke (5% - 15%, just a negotiation), an "industry insider" type (5%-10%, just a negotiation).
Of course these are just negotiations. If the engineer would do it full time for no cash, give him 20% or so. If he wants "$20k to build the prototype" he can maybe only negotiate 5%. If his skills are absolutely key in the field maybe more %.
ii) Actually I can't be bothered giving any more examples, you get the idea.
As I understand it, (2) describes the situation more than (1).
In brief, the bottom line is you're going to want 5-10% or so if I've understood the situation and you're the key partner on the product side making the whole thing.
Again, you've not mentioned at all whether you'll be getting any ca$h as such, and how and when?
Regarding the key questions asked by @Dukeling below
Can you elaborate on how OP is supposed to come up with the $1.5m figure,
The other founds and the OP will absolutely have such plans. They merely have to state their goals, that figure is their goals.
The "sales" person will instantly give a full discourse on how much they he/she expects t have coming in next week, next month, and next year.
or how thinking like this helps more than just saying "I believe I deserve 21% of the company"?
Indeed, all you are saying is "I want 20%". The rest is just words - negotiation.
Thinking in terms of concrete numbers might help, but the "in two years" figure seems like it's almost certainly going to be way, way off - OP might not even know their expansion plans.
The five of them altogether including the OP surely know all this.
Note though that you may be thinking of more "type (1)" companies as on Angel.co
In that case the solution is at hand. You look on angel.co (some 400? similar ecommerce startups presently, use the search) and just pick a number you can negotiate based on that evidence.
The answer is very simple.
First, put aside concepts such as share or percentage amounts at first. Write down what you think the incoming cashflow of the company will be in two years, and what you think the company will be worth in two years. (It's true that for better or worse, some internet affairs are worth money even if they have no cash coming in, but in your case it's a real sales site, so that's great.)
So you've now written down two numbers, how much money is coming in to the company 24 months per now. (So, the amount coming in in that month annualized.)
Great! You're expecting about $40k a month to be coming in in two years. Write down, indeed tell us, what figure you see there.
Can't guess? Ask them.
All the better. They are then negotiating for you.
So, when someone asks us to do one of these, the first thing I ask is "how much cash do yo expect to be coming in 12 months from now?"
So then, how much is the company then "worth"? (As VCs see it.) Let's say
It could be $1m, it could be $20m, but we'll say 7.
OK. So at that point, how much should you get (think of it as a "bonus" - think in cash terms), there and then?
Since you're doing all the software, you're probably going to say
There's your answer.
Regarding shares, mechanisms etc that's for the accountants to work out and it depends what level of organization they are at so far.
State to your partners that
"Two years from now I expect to get shares that will be worth 1.5m given that the company should be worth 7m by then."
and someone else can work out the details and mechanism. Of course, nobody's going to give you 1.5m in two years, they will vest (or whatever) over the years following that (and of course, be worth more as you go along - assuming anyway that this is the 1 in 5000 e-commerce startups that works out).
Until you can state what you think the incoming cashflow of the company will be in two years, you don't have anything to go on.
Indeed if someone glibly says unto you "how many shares do you want?!" or some such, tell them "Hmm, let's have a sit-down and figure out what we think the incoming cashflow of the company will be in two years..."