Hi Mike,
You'll get a lot of different opinions on what works as a thinner for Tamiya paint.
In my opinion, the range of preference is:
1: Tamiya acrylic thinner (X-20A)
2: Isopropyl Alcohol
3: Denatured (ethyl) alcohol
Note that if you use either of the alcohols above, I'd recommend a small amount of acrylic retarder (available from artists supplies stores) added to the alcohol before mixing with the paint.
For gloss and metallic (Tamiya) colours, it's preferable to stick with Tamiya's own thinner.
Don't buy the small jars of thinner - get the 250ml jugs - these run around $10.00, compared to around $3.00 each for the small jars.
You'll see many different opinions if you look around the modelling forums. Everything from water up to lacquer thinner. I tend to prefer to stick to the "keep it simple" rule - use what's been made specifically for the purpose - their own thinner.
Only thin as much as you need for the task that you're working on, don't thin the whole bottle in one go.
You may see people recommending Windex (the window cleaner) - though some people seem to have success with it, I'd rather not use it - it breaks down the paint rather than acting as a thinner.
On thinning ratios, don't get hung up on exact numbers - they are a guide, but only a guide. Paint viscosity, even in new bottles from one manufacturer, can vary from one colour to another and from one profuction batch to another. Viscosity can also vary depending on how long a bottle has been on the shelf, how it's been stored, or, if in your paint stock, how often and how long the bottle has been opened. If you take two bottles of the same paint colour, open one and leave it open on your bench for an hour while brush painting something, then it's going to have a different consistency to the new unopened bottle.
I find Tamiya paints qute forgiving and near impossible to over-thin. I often use more thinner than paint, but that's what works for me and the way I paint.