I don’t attempt to do precise estimates because it takes so long and I find clients would prefer I spend time scanning and organizing instead of counting. The gathering guide I send clients includes estimating tools like ~100 photo prints in a 1-inch stack and ~20 slides in a 1-inch stack. All I want is a rough ballpark to get started.
A practical example is a new client that estimated her content for me. I ballparked the project cost at $6k and asked for a downpayment of half that, at $3k. That way I have incentive to finish the project and get paid in full, and I have all her photos and she needs to pay me to get them all organized and back. So we both have “skin in the game.”
If the project comes in under $6k then the client will “true up” for the difference. If the project seems to be on track to be over $6k I’d inform the client and get their approval to go past the estimate. If the project takes multiple months I invoice monthly so my cash flow is steady and I’m paid as I go.
Hope that helps!