My favorite level now is the green "smiley faced" arc but I don't think it is available except to go around a scope.
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Something to think about, the level mount and/or the pic rail may not be machined 100% perfect. So by using a level mounted to a potentially unlevel surface you could introduce cant. It's much better to use a bubble level mounted to your scope since you know it's level, assuming you took the time to mount it properly (this would also include the spuhr mount. That appears to be a good option but again it depends on perfect surface contact for the base to be perfectly level with the rail). I've always only used scope mounted levels (my current flavor is the accuracy 1st level). But I could be totally off on this as I don't have a ton of experience with your option. I'm just offering some input.
Regardless of where you mount your bubble level, you need to ensure that it is coordinated with your reticle, otherwise it is pointless. Your turrets adjust up/down and left/right in reference to your reticle, so if the reticle and bubble level aren't coordinated, the bubble level is doing more damage than good.
With that being said, I prefer scope mounted bubble levels (accuracy first makes a great level [video]https://www.accuracy1stdg.com/store/itemDetail.cfm?prodID=549[/video]). This allows you to cant your rifle to fit you better (if needed) while still having a reticle level with gravity.
The machined error won't wont hinder anything as long as the scope is mounted to have the reticle and bubble level coordinated. I.E. if the pic rail mounted bubble level is not perfectly level with the rifle, you can still adjust your scope to have the reticle lined up with the bubble level. It works the same way as the scope mounted level, but it just eliminates the ability to level the scope and rifle differently for comfort.