i will try to do this? why do i need to keep the timing at 10deg to do this?

becouse under normal circumstances the timming will swing with every little rpm swing and change your idle propertys. This will skew your results and you could possibly go to far and shut the car off and then go to restart it and find you are way out of whack and either idle way to high or, not at all. If you have ever adjusted your timming by ear, this goes on the same principle.
10 degrees is the standard idle timming for all miatas you could run whatever you want that is just a reference to a baseline we all adhere to.

i see. I will try to do this and see what i get. What idle rpm should i set my MS at 1050?

1k is fine yeah.

so after i set the MS to 1k and the car idles at 1k i will set the timing at 10deg in MS. After that i will start unscrewing the screw on the TB? Would this mess up the idle setting in MS? i mean if its set at 1k and i use the screw to adjust the idle it will show 1k in MS but it will actually idle higher? I am a bit confused here.

nope you just want the shit to hold still while you adjust the air bypass man its not that hard. If i was in your area i would do it in all of 2 seconds and then you would be done.

This may sound dumb, maybe not, but seeing as you have a 90, im assuming you have the 1.6l. Have you checked the dashpot?

i do have 90 and yes the engine is a 1.6l. Whats dashpot?

The dashpot is the little thing on the throttle body that the throttle plate slams against. Its basically a rod that goes into what looks like a vacuum actuator, without the vacuum line. Its job is to slow the throttle plate down when you release it so it won't kill the motor. Maybe there is a hole in that diaphragm, im not sure if there is a real way to 'test' it though, ill look it up and get back on that.