Is there a standard disc brake mount to disc brake rotor distance?
The standards for disc mounting specifies a distance between the rotor and the inner face of the dropout. This distance is different for front and rear brakes.

A 135mm spaced fork sounds like a fatbike fork. Early fatbikes used 135mm front and rear, often specified to allow two "rear" wheels to be used, and swapped in case of freewheel failure or to change singlespeed ratios. Hence, some fatbike front hubs are labelled "RDS", meaning rear disc spacing.

Nowadays, fatbikes with 4" rear tyres tend to use 170mm QR hubs or 177mm thruaxle hubs, and 5" tyres usually have 190mm QR or 197mm TA hubs. The front hub will generally be 135mm QR or 150mm TA, but both would normally be spaced like a normal front hub.

I think front spacing requires the caliper to be mounted 4mm wider than rear spacing, I'll try to find a spec document later.