Jet Engine Bypass Query

[BA 77]

I wasn't sure where to ask this so I'll just place it here.

I've bought "The Jet Engine" book by Rolls Royce and it states that 75% of engine trust is generated by the air going through the bypass duct. I was wondering if that figure falls as you climb (as the air gets thinner) or whether that 75% figure remains roughly constant regardless of altitude.

Thanks, BA 77.