Yes you're all right...but also overthinking it. The best, most balanced make up airs have perfectly aligned plenums that provide just the amount air to supply just shy of what the hoods are pulling out. You should never feel makeup air-it should be-as you say-a curtain that provides an invisible but definitive flow of air over the front (or from the bottom of the hood/appliances) so that anything under the hood is contained and pulled out. Has to be the right amount of negative pressure though.
Should you get a hood with well designed makeup plenums? Sure! Find one that fits in a food truck that allows for wall and ceiling clearance and allows you to THEN have another makeup fan (which is huge-just as big as the fan you'll use for your exhaust) and the required ducting. Not going to happen....
Next best option is to provide fans, evenly spaced, in/on your roof. Is it perfect? No...but does it work? Yes! Will one work? YES! CFM wise...the reality of the spacing is the issue...where plenums come into play...they can take one fan and evenly distribute the suction...which if your 4'10" and don't need clearance than you're ok with building some ducts. Otherwise-grab two fans and place them evenly in front of your hood...it works.
I think its awesome that you're getting some info here...I actually inquired before/during my build and got very little info that led me in any direction...Luckily I've been apart of 7 separate restaurant build outs and have learned alot from hood/exhaust companies and had a great friend from Captive Air work with me on what I was doing to make the best of what we could do. And what I have works. Is it perfect? No not really...but how many professional kitchens have you been in where its been perfect?
And edwmax-with all due respect-yes it does work (having the fans placed evenly with the tissue test). I've even done the smoke plug tests required by Captive Air and while not perfect its certainly workable and within respectable tolerances.
When I say you should have two fans with downblow I'm not saying it should be full blast fans there just to be there. The whole idea is to provide your exhaust the air it is pulling out. Which two well spaced fans in front of the hood will do. All day long.
And just for my curiosity-since I didn't get a good idea when I inquired a few months ago-how do you guys do makeup air? How did you build it into your trucks/trailers? How is it working for you? Are you using AC? Is it keeping up?