I would highly doubt that it makes a real difference whether you do a workout around 50% and one around 60%. Especially in the 30 to 60 minute time range.
I mean just look at Lazy mountain (Z1) and Colosseum -3 (Z2). Both are 60 minutes in duration. 24 and 33 TSS respectively. So 9 TSS difference. For a half hour workout the difference will be even lower. Taku -1 (Z1) 14 TSS and Taku (Z2) 19 TSS. Both 30 minutes. That’s nothing.
So yeah, whatever feels good.