If the hub is threaded (i.e. not a cassette hub) then all you need to know is the overall length of the axle.
If you are loking to make an assembly of 126mm OLN, you need an axle overall length of not less than 130mm, so that you have 1 mm or so of axle with no thread lead-in chamfer to bear yours and your bike's weight in the dropouts. A little more than 1mm would be far better though - that's the absolure minimum.
You can pull the "excess" axle through the cone / washer / locknut assembly on whichever side has the more "free axle" beyond the locknut by releasing the locknut / come from one another screwing the axle through and then re-locing the cone / locknut.
I'd generally recommend that the drive side is spaced as required for the freewheel that you intend to use (and no more) and the extra spacing that you need goes on the NDS if possible.That has two effects - it minimises the length of unsupported axle on the drive side and it minimises the extent of dishing needed in the wheel build.
Lock the drive side with it's spacing up really tight and do all of your hub adjustment from the non-drive side, as the NDS cone and locknut can be accessed when the freewheel is in place, so simplifying ongoing maintenance.
As ever, if you are not sure how to do all of this, refer to someone that is skilled in this area and who can actually see the job in hand ... it may cost you some money now but equally it could save the risk of higher costs later if you make a mistake.
