Quiet sun radio noise is often used for measurements of radio receiving
systems. The radio flux from the quiet sun is a known signal in that it is
measured and reported at observatories around the earth, and therefore
provides a basis of assessment of radio receiving systems.

I have been playing around with a little online calculator to calculate
interpolated recent solar flux data for amateur radio UHF and microwave
bands from published observations.

It is at http://www.vk1od.net/qsrf for those who may find it useful.

Owen