for me it all depends on wind direction - weymouth bay is sheltered from the W and SW if you stay close in - but no good in an easterly.
if its 20mph or more i wont go - 15mph and off the land fine, 15mph and blowing into the bay no-no
ender 10mph any direction i'll go.
Swell and tides also an impact - i went out once in a light to no wind yet there was a heavy ground swell running onto the bay - so we anchored inside portland harbour - lovely flat calm day - only to be met with the same swell later on the way back across the bay - would have been very uncomfortable at anchor in the Bay
Windy App will show you wind, swell and chop!