Location Highway 10 South of Orangeville Ontario.

There are 5 lanes, 2 in each direction and 1 center lane.

At the point where the Highway has a decline, the speed-limit is 80km.

Still on the decline, there is a speed-limit sign "60km ahead", followed after ONLY 90m with "60km begins".

I was driving in the left lane driving around 85km with a row of cars behind me.

I was clicked at 89km per hour in a 60km zone.

I argue, that I was driving 89km and I argue that slowing down from 80km to 60km on a 5 lane highway on an incline without bringing the cars behind me in danger, needs at least 250-300m between the 'ahead sign and the 'begin' sign and that 90 meters is way too short.

Is there any validity to this argument and/or are there any regulations that supports my view?

There isn't even a requirement under the HTA to post the reduced speed ahead signs. And even at 90m, you can see that type of sign even further away than that. By your own admission, you were already speeding at 89 in an 80. Doesn't matter whether you are on level ground, downhill, uphill or passing. You still have to stay within the posted (or sometime not posted) speeds. There is also no "buffer zone" after a sign to allow people to slow down. The speed begins where the sign is.