Well really the EBT works because if that nap is shorter than expected and your LO is tired you are preventing from them getting too OT by BT and encouraging them to catch up on lost sleep overnight to be better rested the next day.
So say you get a short nap and your LO is clearly exhausted by 6pm, rather than keeping her up to 6.30 and having NW and possibly a short night (as often OT babies wake earlier) you are giving her the opportunity to go to sleep less OT and hope she wakes at the usual time, or for us sometimes later. If your LO does this then it really helps prevent cumulated OT that forms with the 2-1 and long A times. However, it doesnt work for all.
1hr40isnt bad, you are nearly there TBH..you could try adding 5mins onto her A time for a week and see if that encourages a touch longer nap. If so then try another 5. (or just go for 10-15min jump)
Great progress!!!!!!