if I am ok with slightly suboptimal efficiency (keeping slightly less tokens than the model supports) ... multiply the length of the result of splitting on spaces by 1.55 for OpenAI and 1.7 for Claude

This sounds reasonable to me. You might also want to consider estimates based on the number of characters. And you also need a fallback for what to do when the user inputs some weird input that doesn't fall inside your safety margin, but instead causes OpenAI API to return an error (maybe in that case you aggressively trim the input and retry?)