Metrics Transformation problem

I have made a fatal error in understanding how < Transformation . Metrics > works. I was trying to add 4 units to the sidebearings of a group of glyphs. I typed in 4 and ran the filter. Instead of adding 4 units, it changed the whole sidebearing to 4 units. Luckilly, I had saved a copy of the latest metrics file so I was able to recover. But, I still would like to know how to do it. FontLab 5 had a selection for “increase by” but I do not see it in glyphs. Is there a way to do this without having to do it one glyph at a time?
55%20PM

That requires the Relative option you see below.

Thanks, Erik!