Pro-rata rights look generous on paper. In practice most angels in a seed round never exercise them, and the ones who do often decide late, after the lead has already sized the round. The result is a cap table where the allocation everyone planned for does not quite match the one that closes.
It helps to ask early. A short note to existing investors a few weeks before a round, with the expected size and timing, turns a vague right into a concrete yes or no. Most will say no, and that is useful information for the new lead.
For the few who say yes, agree a deadline. A pro-rata right without a clock is how a closing slips by two weeks while one small cheque is found.