A quick test has confirmed your post Geo, and destroyed my own assumption. Hmm, it looks like I made a mistake with my maths in that game then, since I could have sworn I was above the ship limit, mothballed ships included. It was obviously not the case in light of this test. Back to Algebra 101 for me!
Thanks for having made that matter clear. One less thing to worry about at least.
(No hidden, mothballed fleet waiting to jump on me)