what is the flux density when there is solar flare-ups?
If you mean a solar maximum, values have been measured at almost 300 (SC19), though 250 has been around the limit the last 3 solar cycles. Solar Cycle 20, from around 1965 to 1976-77, didn’t even break 200 and it was the point at which people were freaking about a new ice age.
That’s the problem with these long threads. I’ll pull together a lot of the data here and put to in a single post soon.