There are companies which I visit on a roughly weekly basis for a few hours to offer consultation about algorithm development, for a specified payment per hour.
Sometimes, in between my visits there's a need for me to clarify / expand on the work we've done in the physical meeting - which we do by email / phone while I'm at my home office.
I'm wondering if there are best practices for how much to charge for this followup work, if at all.
On one hand, this is extra work I'm doing for the company, and as such should be billed normally by the time spent.
On the other, this usually takes a relatively short amount of time, and I don't want to be so stingy that people can't even talk with me without me demanding payment. Also, depending on the specific circumstance, there might be a case for arguing that this is a form of warranty I'm giving for the work I've already billed.
ETA: I don't have any written contract. For every client I just presented my fee, they agreed and we started working. So I'm trying to understand what would be considered implicit without a contract, or generally accepted and appropriate.