The +3 pad was initially designed for the first gneration of STI 40 caliber tubes which required a 13mm pad to extend them to 141.25mm. On those mag bodies you should be able to get 21 rounds and stay with in the guage.
The +3 pad will work with your 140mm open mags and should allow you to get one more round or load more easily to the magazine when it is topped off. The developer is working on a pad to make the 40 caliber, current production, 140mm magazine max out the guage at 141.25mm. This should be out in a few weeks. He is also looking at producing the basepad in stainless steel to give it added weight and additional durability.