Framebuilders - good BB height for short cranks?

I've been looking for a bike for my wife, who is pretty short at about 4'10", but fairly normal proportionally.. and even though we're in Taiwan and ther are plenty of smaller frames around, none ft her in terms of a decent top tube standover height. even the small wheel offerings from Giant like an OCR W in XXS size are too tall in the top tube, which is just ridiculous seeing how low the seats are on them.

i finally realised the reason: almost every bike i saw with 650c wheels, in months of casual looking, has had 170mm or in one case, 175mm cranks installed! of course, this means a really high BB is needed, almost at the level of the axle, and even then there is minimal safe pedal clearence. and as well, how on earth is she meant to turn a 175mm crank, when that's longer than I use on my race bike?

So, i think we might be investing in a custom frame, perhaps with a more reasonable 150mm crank set up (stronglights).

Any one out there with some idea of a useful BB height for such a frame, given that we will be looking at building a short top tube (not very long arms either), and we must have a decent top tube standover height. I am thinking a compact frame with sloping top tube will help. there is a guy who makes Ti frames for reasonable prices here, so we may go that way...

we don't want a mixte type bike, which have the wrong geometry, but a capable road bike light enough for her to pedal uphill and sit on for several hours... and one that's not built from a large crank, large wheel perspective.

I just build frames for myself, and when factories build frames they don't worry about smaller riders or alter BB height always. if I was going to do what you are suggesting, and presumably any competant builder would have experience, first I would look at typical BB heights for frames such as you are planing on getting. Depending on use you will have different BB heights. Such issues as ground clearance over rocks, and handling at low speeds can require more pedal clearance. Then I would feel confident in subtracting the length of the crank from a standard 170, which would in your case give you about 20 MM lower BB height. However I would probably trim that amount by 25% unless you are using small pedals also or clipless that are within the bounds of the foot, and trim or contoured. In the example given that would be 15 mm. You could also take a little off if the frames you are using as an example have the same BB for huge sizes where 180 type cranks would be used, and yet apparently still have clearance. Back to 20 or 25 mm in this example.

Overall though, I wouldn't push the issue of ground clerance in order to get more standover height. Attack the problem where it is actually hurting you with the top tube. Look at a Bike Friday and how low a mono tube can be while still retaining some performance. You may not want a Mixed frame, but that tube arrangement with the top tube sloped below the seat stays is found on a variety or performance bike these days. The main thing is that you are happy with the bike so just put yourself is a well known builder's hands, you should be taken care of.

Ta... Bike Friday, hey?