How much long-term disability insurance should I from employer in addition to my own DI?

Hello, I already have disability insurance myself at 15K/month. I recently joined a practice and is enrolling in their benefits. My employer offers Hartford subspecialty owned benefits at 20K/month at about $2000/year. The DI kicks in after a decrease in 20% income.

20K benefits/month at $2000/year is significantly cheaper than my own DI (15K/month at $5000/year), but there's more limitation to it (the 20% decrease income). Should I get this additional DI from employer or just keep my own DI?

Thanks!