Hiya Amy. We were having exactly this issue with O only a week ago, after I pushed him to a set nap over Christmas. Before Christmas, our routine looked a bit like the one in your original post except for a 6/6.30pm BT & I wanted to shift everything an hour later, which we did gradually, but he kept up with the EW, even though his BT was an hour later.
I realised he was mega OT b/c I was keeping him up til midday for his nap, even if he EW. Some days he had to do 7hrs A to nap. So one day last week after another 10hr night & 5.30am WU I decided enough was enough. That day I put him down for his nap early at 11am & let him sleep 2.5hrs & did a 7pm BT. He still did a shortish night, but was a bit better rested the next morning. From that day on I've stuck to 5.5hrs A like glue, & capped his nap at 2hrs & done 5hrs A time to BT. And his WU got later & later & for the last 5 days we've had a 7am WU, a 2hr nap 12.30-2.30 & a 7.30pm BT.
So I would def agree with PP's that a 7pm BT is too late if C is starting her nap before midday for 1.5hrs. Like koe2moe says its more the length of her day than anything else thats going to cause the OT - exactly what's just happened for us.
IIWM I'd probably try something similar to what we've just done. Stick with 5.5hrs A time absolute max in the AM, and 5hrs in the PM if you get a 2hr nap. 5.5hrs if its 2.5hrs+. Or if you prefer to keep the 12pm nap I'd def try an earlier BT, at least for a few days, to keep ontop of the OT that will arise from being pushed in the mornings. I'd also def try to keep the day nearer to 12/12.5hrs long at most.
HTH.x
