How hot should the exhaust manifold be with 312 Y block warmed up at idle ?

My 292 are all in the high 300's to low 400's I check them right on the the short spot right at the head. My engine has dual quads and idles off both and runs or rear only.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

I'll try on the same spot. I've been testing in the middle of the exhaust manifold that corresponds to the piston and the one closest to the firewall.
The temp is +540 on left and +520 on the right. 
Next time, I'll be trying your method.
Thanks for your time and in put,
Tim 
Florida