[Occasionally6]

Understanding what you are trying to do is more important than any specific method. Find a BSFC contour plot - any will do, they're all much the same basic shape - and look for the parts of the island with the highest thermal efficiencies. You are trying to operate the engine, when you have to, in those areas for as much of the time as is possible and not all or very little (= coast) when you can't be in those islands.

If you look at the axes that reference the islands, the rpm range and load range that you want to be in should be clear. You might have to convert the axes from a unit of absolute pressure into inches of Hg if that makes more sense to you.

I do prefer to use kPa on a vacuum gauge though because "0" vacuum is close enough to 100 kPa that I can view the "desired" manifold pressure as a percentage i.e. in the range of 60-80% (~kPa) of maximum.

Throttle position doesn't correspond directly with inlet manifold vacuum. The other variables are the load on the engine and rpm. You have noted that you can hold the throttle and rpm constant while climbing a hill and see the manifold vacuum fall.

That's good because the load is high(er) and towards being in the high efficiency part of the island. If the load gets too high - manifold vacuum gets too low - and you start to fall off the other side of the thermal efficiency peak, back the throttle off (allow your speed to reduce) or change down a gear.

Trying to hold a particular vacuum reading i.e always at the peak of the efficiency island, is not possible without varying your speed so much you will impede other traffic. There's not all that much to be gained over being just close to the peak anyway.

Always (neutral, or better is engine off) coast up to a red, unless you have to brake, in which case, stay in gear and motor the engine. Try to avoid the braking part though.

Even on a highway, constant light load is worse than some variation of pulse and glide. The P & G can be subtle.