Custody Help😊

Hello ladies! I am new to this group and just have a couple questions. So my Husband and I get my stepdaughter 2 days a week and pay over $600 a month in child support and in their mediation plan it states that they can renegotiate custody at any time if they can agree on it. His ex will not let us have her every weekend as they have trips planned out for the year on those dates. So what would be a fair amount of time to ask for... she will not do 50/50. We were thinking Wednesday , Thursday , Friday every week? But we would also want her at least 2 Saturday’s out of the month. We’re trying to settle this outside of court as much as possible. Thank you loveys in advance!