So I'm replacing the triple crank on my bike with a proper single chainring crank. The old crank is an ISIS and the new one is square taper, so I need to buy a new BB (which is fine 'cause square taper BBs are better anyway).

Problem is that I don't know what spindle length to get for a perfect chainline. I don't even know what ballpark to use and even if I could make a reasonable guess I figure this is my chance to straighten out my chainline without resorting to spacers on the chainring.

Is there a procedure one can use to figure out the correct BB spindle length when you have the crank in hand and you know what the rear chainline measurement is?

What crank will you be using?

Yea, we could help you better if you provided what crankset you are using.

Usually, 107-110mm JIS bottom bracket will provide you a nice chainline for a double crankset.

Odyssey Black Widow Euro Lites. I found this page https://www.recumbents.com/wisil/qfactor/qfactor.htm
and derived the following info from it along with some measurements:

BB=Bottom Bracket Spindle Length
BWQ= Black Widow Q-factor (ie distance between pedals)
BWQ/2=Black Widow Q-factor divided by 2
CR2P(out/in)= Measured distance from chainring to pedal (outer position/inner position)= 23mm/27mm
CL(out/in)= Chainline (outer position/inner position)

BB: 118mm, 107mm, 96mm, 61mm, 53mm
BWQ: 141mm, 130mm, 119mm, 84mm, 76mm
BWQ/2: 70.5mm, 65mm, 59.5mm, 42mm, 38mm
CL(out/in): 47.5mm/43.5mm, 42mm/38mm, 36.5mm/32.5mm, 19mm/15mm, 15mm/11mm

Formulas:
CL(out/in)= (BB+23)/2-23 or (BB+23)/2-27
BB(out/in)= 2CL+23 or 2CL+31

Wait, is this going to be going on a 'bent?

And 107mm should give you the closest chainline with the chainring on the outside if you are using a track hub.

Shhh, don't tell anyone! Bent riders are hostile to the finer points of gearless drivetrains and SS/FG riders are hostile to bents!

Fair enough.

I have no experience dealing with 'bents so I don't know if the said spindle length is right for you.