And it's important to say that the SWR may change based on where you place the mag-mount antenna. If you put it on the roof one day and on the trunk another, it'll be different.
I just borrowed my pal's SWR meter to tune my through-roof NMO mounted antenna in the rally car. I had to trim 1.5" (!!!) off of the antenna to get a 1.1 SWR reading. This was because the antenna USED to be on a mag-mount and is now effectively 1" lower.