I'm after some general guidelines as to finding the optimum stem length on my Trek 520.

I recently bought a used 520. The 520 is the first drop bar bike I've owned. It has an ETT of 555mm. I've currently got a 110mm stem and Nitto Randonneur bars on it, so it has a combined ETT/stem length of 665mm. The tops are pretty much level with the seat.

My other bike is a flat bar road bike with an ETT of 570mm. It also has a 110mm stem and bar-ends. The combined ETT/stem length is 665mm, this being 15mm less than the Trek. I ride on the bar ends a lot. The flat bars are about an inch lower than the seat.

Now here's the thing, the 680mm combined ETT/stem length on the flat bar bike feels about right, but the 520, at 665mmm, feels way too long. Riding on the hoods has me really stretched out.

Riding in the drops feels OK, with about an inch of clearance between my knees and elbows, and is much more comfortable than riding on the hoods, but I prefer to sit more upright.

So, is there any magic formula for arriving at a combined ETT/stem length for drops versus flat bars? Or to put it another way, assuming you were working with the same road bike frame and changing bars, would you normally change the stem length, and if so, by how much?

Should the distance from the seat post to the hoods on the 520 be about the same as the seatpost to the flat bars on the other bike, or is there some other guideline O should use?

I don’t have a supply of stems to try, so I’ll have to buy and try some. I’m just after a ballpark estimate of what length I should be looking at.

Thank you for reading.