ATM secondary advice

["alderlie" <alderliesten.atelier@worldonline.nl>]

To have optimal use of my 20 mm, and 7mm Nagler, I do need
some advice.

I have a 16 inch, F6 ( f 240 cm ).

What is the best secondairy mirror size, and focuser range,
for these two Naglers, 

Thanks in advance, Kees