I have no idea what the fig is for whatever PA28 you fly but it is not hard to work out. I think my old DR400 was about 105 seconds, which is still insignificant if you have planed you PONR and know which way to turn without wasting too much time.
Consider flying Dover VOR to ING NDB, it could reduce your risk window by a considerable %, you have a nav aid at each end and less chance of banging into anyone. Your route would put me out of gliding range for about 314 sec, which is a huge increase on 90.
The safest way to cross is one where you do not end up in the water at all. The PONR calc and the how long calc are well worth the effort. It is also an excellent way to understand the minimal risk the short crossing represents in most aircraft. I did the calculation for a Nipper once, and stayed at home!
Rod1