Grill Blocks
 

Is there a general rule as far as how much space you should leave open when blocking the grill on a car? I recently put on an upper grill block on my car and have now decided to do a lower grill block. I would eventually like to do underbody panelling too, so I probably won't be getting much air intake from underneath. Regardless of what I do, I'm still going to test it to see if the engine temperature goes too high. I just want to know if there is a general rule.

I've never heard of anyone having issues with blocking their upper grill block year round (except perhaps Jeeps). I at least run an upper or lower year round and a normally a full block in winter unless I'm going on an long road trip.

On some cars, the upper grille is purely cosmetic. Blocking the lower grille will require that you monitor the coolant temperature via a scangauge, ultragauge, etc.

Ideally, you want to optimize the opening to get adequate cooling. The opening will vary from one car to another. An accepted rule of thumb is to leave the middle of the lower grille open and block the sides.

You can get a good cooling airflow with an even smaller lower grille opening, but it will require ducting to the radiator.

To give you an idea of optimization, my current lower grille block was not efficient at speeds higher than 30 mph. I added a small coroplast deflector which redirects the incoming air towards the radiator. It has since then been working as it should, keeping the coolant temp stable at 188F, making it efficient at any speed. Trial and error experiments are usually the way to go when installing a grille block.

There isn't.
Usually you're OK to block the upper grille, and have all cooling done through the lower openings.
If you want to go even further, cover part of the lower grille, and then progressively block a larger part with something you can easily remove if needed.
Find out at which temp your thermostat opens.
Watch the temperatures closely when doing a lower grille block - preferably on something more accurate than the built-in temp gauge.

I know my fan kicks on when coolant temps hit 217F and with no grill block my temps usualy hover around 195 in the summer (90F+).

I like to run mine so it averages less then 210F. If the fan kicks on you are running it too hot.

As others have said, just be careful. I would recommend that your first attempt be temporary and easily removable.

Also, a belly pan won't really affect air entering your radiator.

Some cars are more sensitive to a decrease in airflow, in my sucky '96 Chrysler the temp will skyrocket after driving for a while then coming to a stop like at a red light for bit. When I do do a grill block it will be an upper one and small.
I've been trying to think of a way to make a grill block so I can open and close it while driving, it would be ideal but difficult.