You need to make the first move. Trust me. I didn't, and it took me $20,000 of lawyer bills to get from the standard Texas possession to the oddball possession I have now. If you file first, you are instantly ahead in the whole proceeding.
EVEN if you are going to do this amicably. You can always mediate and negotiate during the process to 50/50. That's all I want, and my ex-wife only gave me 45%. I took it, given the risk was too great to go to court & risk more child support & no additional time with them.