the dimension your are trying to achieve, in the USA, in the curtain wall industry, is called a DLO (daylite opening). DLO results are a function of the center of mullion to center of mullion dimension (called a MD (module dimension)) and the profile width/shape of the mullions being dimensioned. In your example if your left 3'-0" MD witness line was to the center line of the mullion at the corner then the DLOs would be equal. Or, IOW, to achieve equal DLOs you need to move your mullions to the right by 1/2 of the mullion width (1 1/4")

Edit: I just noticed that you have a jamb on the right. If your opening width is fixed then you need to take the opening width dimension, less 7 mullion face widths, divided by 6 to get an equal DLO of approximately 33.083" or a mullion MD spacing of prox 35.583". Most people in my business (glazing/curtain wall) just let the end DLOs just be what they may. When you get to the field and measure the opening it won't be 18' wide in most cases anyway

The simple answer is.......

Just check the parameter called "Adjust for Mullion size"- this is SPECIFICALLY what this parameter is for.


Kind Regards,
Ian

The Complete Beginner's Guide to Autodesk Revit:
www.bimscape.com/revit

works great...except at corners as shown in the OPs screen shot

Just mocked up both situations and it seems as if the "Adjust for Mullion Size" does work - or an I missing something?
nonadjusted.pngadjusted.png

sure, you can force it to work with a corner shape that is rarely used in real curtain wall life. In my life corners are rectangular (parallel face width equals perpendicular profile depth), arrow shaped (SSG glazed), arrow shaped (captured glazed with a face width that is different than at typical mullion), or non existent for glass to glass SSG corners joints. In my mind, if the OP is going to worry about getting equality in DLO dimensions then he/she should also worry about real mullion and corner profile shapes. Or, better yet, let the curtain wall detailer worry about it, it's what we do

Absolutely its the detailers job: but the OP still needs to get it sort-of right, especially if internal walls are coming off the mullion centers etc. Using "Adjust for Mullion Size" makes it, err, more sort-of-right?

well, not to kick a dead horse over the cliff...but, assuming that the OP is an Architect and he gnats a** figures out where 7 equally spaced mullions are so that interior walls will align with mullion centers, 35.583" oc in the example above, and all DLOs are equal. Great! Then the steel stud installer arrives on his project prior to curtain wall installation and his acceptable tolerance for cl of stud placement is +/-1". Given that what's the point of "sort-of-right"? This is why an 8lb sledge is standard issue equipment for glaziers ;-P

and in closing, for what it's worth, in my long career in the glazing business I've never, ever seen an Architectural curtain wall plan layout done by equal DLOs. 98% of the time if there are repetitive equal modules the layout is done cl of mullion to cl of mullion. Given standard mullion face widths that gets one equal DLO sizes until you reach a corner or a jamb condition.