so your LO's current day as posted is:
WU - 7.15/7.30 am
Nap - anywhere between 1.45 - 2.00 pm (sleeps between 1 hr to 2 hrs but usually 1.5 hrs
BT - 7.00/7.15pm or 3.5 hrs after nap
Usually sleeps 12 hrs but less if later to bed!
So I'd shoot for something like this:
WU: 7.15/7.30
nap:1.45/2pm for 1-2h
BT: 6pm (1hour nap)
6.30 (1.5h nap)
7pm (2h nap)
I am only suggesting the above because you say that the nap length is so variable. TBH, if you manage for hr to have one full nap as she needs it, I would try and set nap and BT the same for each day. I know for my DD if I were in your situation, that is what I would have to do. Also, it helps regulate LOs sleep patterns (once they get over the OT). Its a tough one really as setting naps for a 14m old IME is quite unusual.
Hth xx
ps. have a look at this link, I think it might help answer some of your set times questions:
Set naps for toddlers: Why, How and WhenH x