Random question about tamiya acrylics

As someone who has worked on industrial tools I can tell you that the machine puts the exact same amount of paint in each bottle on the rare occasions that it doesnt i cant speak for. I Would advise you not to mix any chemicals together that you have no idea of the reaction.  SECond putting thinner into paint in my experiance shortens its lifespan i may be wrong but that is just what i have learned

I bought a ton of clear plastic pipettes on eBay a while back for under $8 and they are reusable multiple times. They eventually separate at the top seam line at the bulb after a good 50 or more uses specially when using lacquer thinner. 
Thinning paint depends on the airbrush being used, tip size etc. so I thin on each application.
A fantastic thinner to use with Tamiya paints is Mr. Color Leveling Thinner.

Hi;

 When I was working in Environmental Cleanup ( Marine Pollution Control ) I was given a test kit by our prime contractor to test waters with, where the oil got ashore. It had some Glass-Laboratory type pipettes. I have since gotten some from a pharmacy too. Why? I used a lot of Lacquers years ago. They would dissolve the plastic ones. 

Just a side note on the pipettes:  Up until I started using them just recently, I was convinced they would only be good for a single use due to my incorrectly assuming they're not resistant to strong solvents.  The first one I tried, I did an experiment to see if it would tolerate being flushed with MEK.  It was actually able to handle it without turning to goo, so in addition to being cheap, they're also somewhat reuseable...something I had assumed wouldn't be the case.

I'll probably continue using my glass syringe for mixing MEK into my primer though...I have a good system established for that.  But, the pipettes are awesome, and not as clumsy as the party straws cut in half that I had been using by capping the end with my finger.

When I thin Tamiya for airbrushing the blend is someplace between 45-55% thinner so no way are you fitting that much thinner into just the neck of the bottle. In fact even brush painting I use at least that necks worth of retarder if to address the entire bottle for brush painting. Best bet get yourself some pipettes so you can properly measure your mix per application and leave the paint in the bottle pure paint. You can get 100 pipettes from Amazon for $4.79 on prime with free shipping or $2 at Ebay but + shipping.

For brushing Tamiya I drip some paint on a painters tray and mix retarder into it till it loads a brush nicely and flows off smoothly.

I do this usually myself although I pour thinner so the bottle is just short of being full works fine so far I've not had an issue and same goes for Mr Hobby colours. Haven't tried it with any other manufacturer but if you do this be sure to write air on the paint jar as it'll be not much use for hand painting after.

Thank you for the replies.. I guess I'll just thin as required when I switch over to them... 

Just checked a brand new 10ml bottle of Tamiya X-31 that I got on Friday and have already taken a tiny bit of paint out of it for brush painting...it has paint all the way up to where the neck of the bottle starts.  The brand new 23ml bottle of XF-1 that I got the week before has paint up to just above where the neck of the bottle starts.  Also have taken a little bit out of that already for brush painting.  Nothing has been added to either bottle. 

Also, note that proper thinning also depends on your particular airbrush and other factors.

The theory is that , if you open a new bottle and add tamiya thinner, up to where the bottle tapers (basically, where the threads start), it should be thinned well for airbrushing. Don't know , never tried it. I never thin the whole bottle.

I would say that its not true.  Every Tamiya bottle I have had has always been filled to just slightly below the neck.  Not enough space left to get it properly thinned for airbrushing.