yea sumtimes you have to try out different stuff in order to find the sweet spot..
but it sounds a bit wierd to me unless your MH is on less Watt the your HPS ? I was in the impression that MH got hotter ?
and I agree 81F is`t bad just keep some good air circulation in there with a fan or sumthing .. cool the hood from the outside aswell
but if you want them perfect 78F you could try to run some dutching to a cooler room/window etc. to "collect" some cooler fresh air for your intake hole/holes and/or try to lower the ambi temps of the entire room your tent is in ..
and no matter what, you will need a bit of negative pressure in there or smell might leak, specially with a DIY tent, with a bit of negative pressure you ensure all air going out, is through your filter, if you turn your vent off and then on you should see the sides be pulled in a inch or two, tho normally rule of thump is that the intake holes need to be between 2 and 3x the size of the outtake fan, agin sounds weird, I have 3x 4" for my 6" vent on a coolhood and with a filter on in flowering