Just say "I use a third-party client to track my xp/hour. I am a hardcore skiller (or if you're doing combat, just say you like to know how long to the next level, etc), and physically seeing my numbers is such motivation for me. If this is a problem, I will receive punishment but don't be too harsh on me, as I am just ambitious!"
If they come at you with physical evidence of botting, oops! Otherwise, how're they to tell you're lying?
Jagex is pretty lenient if you're "honest". One day I noticed my membership renewed when I didn't want to. I was over 24 hours into my "next month" of my subscription. I messaged them, said this isn't what I wanted, and they cancelled my membership and refunded the monthly fee. They're nice to their honest, dedicated player, because they don't want to lose your business.
TL;DR - My client is for XP tracking, not botting, release your own tracker or leave me alone