recursive-clients : recommended value for a high traffic recursive nameserver

[blrmaani]

Our nameservers take upto 10KQPS (mostly NOERROR type most of the time). 

Twice or thrice a week, I have seen upto 10% of the queries are SERVFAIL and we have started exceeding the default value of 2000 for recursive-clients settings in BIND 9.9.x.

Is there a recommended value for recursive-clients option assuming huge number of SERVFAIL queries once in a 2/3 days?

I'm not convinced to increase it to some arbitrary huge number 20,000 or 200,000. 

I am looking for answer like - if your peak SERVFAIL queries are 2000/second, then your recursive-clients value should be N.

Please help!

thanks in advance
Blr