You have all heard of the stem fitting guide ( if the front wheel axel is behind the handlebars, the stem is too long, vice versa).

Does the stem angle affect the the handlebar/axle correlation?

For example, assuming both stems are the same length and correct length, will a 6* stem and 25* stem both have the handlebars over the front wheel axle?

That's a good rule of thumb for a bike, but what fits you best is vastly more important. But no, it will move it back and forth a little.

Agreed. I recently swapped the stem from a 90mm 7* to a 90mm 35*. the difference was about 34mm higher and 25mm rearward. this still kept the bars over the axle but moved them back just a bit. for me (a guy with short arms) it worked perfectly.

what helped me was comparing the old and new stem bar placement on an online calculator. it gives the exact change with the proposed stem.

You're asking the wrong question.

The answer to the right question is that the axle/bar relationship isn't a useful fit guide.

Like gravity, they keep the geometry on at weekends.

It a convention that doesn't apply to many. For example, this convention has no more veracity than KOPS. Neither apply to me...I like the handlebar in front of the hub and I have ridden with every combination.
You can perform a simple experiment. Test the handlebar covering hub convention both on hoods and in the drops...it changes.
What that means is your bar height matters and your visual viewing point depends on your torso angle...and look how differently people ride in terms of handlebar height...no convention there...you just recently going for a riser stem. For me, the higher the handlebar the farther the bar needs to be forward to preserve reach. A guy will longer arms like me will need the handlebar more out in front than a guy with short arms.

Yeah, that system for measurement is really wrong.

IMO, the better way for establishing handlebar placement is the position in relation to the bottom bracket - or at least that is the measurement I use for setting up different bikes.

Really, you handlebar position should be determined based on your flexibility and fitness. You are looking for a position that maximizes power and aerodynamic efficiency. It seems to me that too often people are quick to lower their bars for appearance and aerodynamics, thus increasing the hip angle, without considering whether the position is too low, robbing them of power.

No, the stem with more rise is going to have less reach.

Handy on-line stem calculator: https://www.brightspoke.com/t/bike-stem-calculator.html

The bar covering the front hub is awful way of doing it. Considering how you can have almost 20mm reach difference from one bar to another you need to adjust the stem size to match. The reach to the hoods is all that matters and then if you can go into the drops without discomfort.

Bar position is not a matter of fit; it is a matter of body position preference, aerodynamics, and bike handling.

Its a sine of the times!

I agree the old wives' tale of bar covering the front hub is not a hard and fast rule. But like other guidelines, it gets you in the neighborhood.... a good starting point if nothing else.

Kinda like KOPS.

You are going off on a tangent...

I agree. But usually all fitter will take what you want and fit to the hoods as position 1. All I was pointing out was that there can be a big difference in reach based on the model of bar you pick with the same size stem.

I think you are being obtuse.