What gauge string should I get for higher tunings?

A half step up won't make any difference unless you're already using large gauge strings.

 

To be pedantic, a semitone up does make a difference =) Tension is multiplied by 1.12, a 12% increase. Not a lot and not dangerous if you are using a medium gauge set. But if you want to preserve the tension you prefer, which is obviously desirable, just use the next lighter set available, which is typically all gauges decreased by .005, this will approximately cancel out the tension increase.

 

I'd go for a 40 - 100 set

 

Considering your bass is a short-scale, I wouldn't worry about "excess tension on the neck", keeping in mind any given set of strings would have less tension on a 30"-scale than on a standard 34"-scale, even when they're tuned up half a step.

Your main concern should be finding a set of strings that fit properly on the thru-body Mustang you have. Most "short scale" strings would be too short to fit on a thru-body Mustang, which actually requires "medium scale" strings by most manufacturers' specs with only a couple of exceptions.

 

Ah, sorry, i did not read this properly. If this is your first short scale bass then you have no idea what gauges you like for EADG on such a scale.

A good way to guess is to use a tension calculator to discover what string tensions you like on 34" (or whatever scale you have experience with), then reset the calculator scale to 30", reset tuning to F standard, to discover the gauges that recreate that tension on 30" in F standard. Then perhaps adjust to slightly lighter gauges to compensate for how a shorter scale makes a particular tension 'feel tighter'.

 

Better answer:

Tension equation:
T = (U * (2 * L * F) ^ 2) / 386.4
With all else equal:
T is proportional to L ^ 2
T is proportional to F ^ 2

34" -> 30":
T multiplied by (30 ^ 2) / (34 ^ 2) = 0.7785

Tuning up 1 semitone:
T multiplied by (2 ^ (1 / 12)) ^ 2 = 1.1225

Combined effect:
T multiplied by 0.7785 * 1.1225 = 0.8739

When going from 34" EADG to 30" F standard, tension falls to 87% of its previous value. So any string set suitable for 34" EADG (.050-.110 maximum) will be safe to use.
Because a shorter scale has a higher 'perceived tension' for a particular tension, i suggest as a starting point using the gauges you prefer for 34" EADG.