You simply double-click any dim to add-to or override the text. If you decide to create a style for this then I'd recommend a suffix instead of prefix, it's more common in drafting standards. A symbol for what you were looking for on the square would a perpendicularity symbol, which could be found in the GD&T font by exploring with the Character Map in Windows. I don't think I would mess with that method of dimensioning this feature though.
There is a bit of info missing from your sketch, and I have to admit that in order to get all these dims in it would require breaking some "best practices". Some dims would have to place upon the part but as few as possible!
Here's a sample of my approach...

The missing details are assigned "?" marks.
If you provide an overall horizontal length and remainder from 60mm to right you would not need the angle of the 100mm line. Any two of these elements negates the third.
After setting up my 2 dimension styles for horizontal text and aligned text I simply added centerlines for the 3-hole pattern, then I dimension-ed everything. I had to turn off extension lines (arcs) and centermarks for certain radius dimensions, in Properties. I also added Dimension Breaks for places where extension lines and leaders crossed.
I will say that your attempt looked pretty clean but I always try to keep as much info off of the part as possible to keep with best practices and standards.
Hope this helps.
Blaine Young
Senior Engineering Technician, US Army
