The short answer is it really does depend on the project. There isn’t a universal budget ratio because no production is exactly like another. I personally wouldn’t approach a budget that way because then I’d essentially be guessing - rough percentages don’t have anything to do with how much things actually cost. I’d break it down through my experience from previous shoots and quoting/contacting a bunch of people to find out actual costs so that I’d get realistic totals.
This rough ratio breakdown of pre-, production, and post could work when you have years of experience and you need to communicate quickly…but I would only trust really experienced producers to make those calls.
Your concern makes sense because we see this happen a lot - people want to throw money into production to get the most expensive equipment and post is an afterthought. 5-10% of your budget might totally work or it might be low for what your post team will actually need - you won’t know until you start applying actual figures and doing the math. Doesn’t sound like you’re the producer on this shoot but if you’re worried about it, you could put together some numbers together to rep what you think is adequate for post or voice your concerns. Always helps to say something in a constructive way than get to post and realize you have no money. Or worse, you have to drag it out and the movie never gets finished.
Hope that helps a little. Either way, hope you guys have a successful shoot!