Mag gain from sliding EP out of barlow?

A question for you barlow experts out there . . .

I understand from several sources that you can obtain additonal magnification from a given eyepiece by backing it out of the barlow slightly, thereby eliminating the need for closely spaced eyepiece focal lengths. Is this the case?

I generally use a 6" f/5 scope with 2x Ultima and 3x Antares shorty barlows. If I back out an eyepiece (say a 12.5mm or 10mm backed out 10mm/1cm), is there a way to determine the approximate magnification gain through this method.

Thanks for any advice you can provide.

Yes, that is correct. The further away the eyepiece is from the barlow lens, the larger the magnification. Also, the more likely that you won't reach focus.
If you are using a dob, you probably will not see much of a change from a few mm shift as 1) generally the rule of thumb is that a 2x barlow after the diagonal gets you about 3x before the diagonal (and you don't use one) and 2) you likely run out of focus travel before much change happens.
You'll just have to try what reaches focus and what the difference is. Did you look at, say, the moon and try it out?