are you saying the 10 or 15 should be your lower power?
The lower power doesn't really matter much (to the point that using a fixed-power scope is also fine).
I have shot F-Class with a fixed 15X Weaver, a 6.5-20X Leupold and an 8..5-25X Leupold.
- the 15X Weaver worked great and I shot some really good scores with it, but it felt *small*.
- on my two variable Leupolds, I would almost always shoot the entire match with the scope set to the maximum available power and I would still be wishing for more.
Sometimes when setting up a rifle before the beginning of a match, setting the scope to its minimum power can speed things up by a few seconds (because at min power you get a larger field of view, so you can more quickly get rough-aligned onto your own target).
On occasion I'd shoot a sniper match, and oftentimes it's safer to shoot with your scope set to 12X rather than 20X because the bigger field of view is more important than having a bigger looking target. That said, I have been beaten in more than a few sniper matches by somebody shooting a "wrong" scope like a 36X benchrest scope (he won the match because he was overall a better shooter, not because of his scope).
I haven't shot one but I have looked at one in the gunstore and the $1000-ish wide-range-variables (eg 6-30X I think?) Bushnells look pretty decent, I'd certainly give one a try. Or one of the $800-$1500 Leupold target or military scopes, fixed or variable power. If you have money to spare you could buy a Nightforce, you don't *need* to go to that level of scope but then again you would not be needlessly tossing money around either.
If you have hardcore tactical sympathies you could shoot with a fixed 10X scope and you will be able to achieve at least 95% and more likely 99% of the score you'd be able to get with an all-out target-shooting-only-scope.
If you want the ability to shoot at 800 yards or beyond at some time in the future, then one thing you should look at is the amount of elevation adjustment available in the scope. A great many otherwise excellent target scopes partially or fully cripple themselves by offering only a modest amount of elevation adjustment range. If you only need to go up to 600 yards then any scope will do you just fine, in this department.