I work full time as a software developer. A friend of mine runs his own software development company (in a completely different sector to the one I work in). My friend needed some help getting a project delivered by a deadline and asked me if I could do some work for him. With my own manager's permission, I did so.
We agreed an hourly rate, I did the work, and he was happy. All is good.
However, he is a friend who has a small company, and I was not desperate for the money, so I neglected to invoice him. We both knew there was an invoice due, and he indeed asked me several times to send him it, but I waited quite a long time.
Here's where my mistake became apparent. I at no point made any log of specific time I had spent working. It is now 9 months later and he asked me again to invoice him, so I decided finally to get on with it and work out how many hours I have worked for him.
Since I did not keep any time sheets (yes, I am aware this is a major mistake, lesson very much learned!!) I had to use what evidence I could find in order to calculate hours done.
I could calculate with some degree of accuracy the time I spent coding, because I am of the "commit little and often" camp, and so his version control system had logs of all of my commits, which I could query. I calculated my hours based on these logs and came up with a figure. However, there was also a lot of other time spent planning, having meetings / discussions etc. which are not so easily quantified due to my poor record keeping.
I sent him an email yesterday detailing the hours I calculated from the version control logs and followed it up with a phone call in which I explained that this was not the total, because there was a lot of extra time spent, but I could not quantify it.
He is a very reasonable person and a good friend and he said (exact words) "How much extra would you like, is 10% okay?" I told him I would have a think about that and get back to him very soon and he seemed happy. He then said "If you think it is more then just say what you think, but don't go crazy"
I actually feel that the extra time spent was a lot more than 10%, but without evidence I feel like I would be cheeky if I asked more than the 10% he offered.
My question is this: How can I maximise my invoice to him, without alienating him? Given that we are on good terms and I expect to get more work from him in the future. Should I just accept payment for the time I can provide evidence for plus the 10% he offered for planning/meetings. Or if I was to push for more, how would I go about it?