If you definitely want to buy premixed base, then all I can suggest is buying a higher VG/PG ratio than you intend to finish with.
E.g. buying premixed base at 85/15 and adding 15% flavouring would give you a 70/30 ish finish…but, your nicotine is still going to be slightly out (it would end up being 2.55ish mg/ml strength
You’d still have to manually firk about with the “desired strength” box to prevent the error message flagging up.
Is there any particular reason you want to go down the premix route? If you aren’t too fussy about the finished strength being off-kilter then it will be fine, just not as precise as it’s possible to get it.
Am I making sense? Lemme know if not