Another, quite accurate method is the 'Rule of Squares' taught in the RAF (a long time ago):
To get the percentage of IAS to add to get TAS use the following:
5000' 3² = 9%
10000' 4² = 16%
15000' 5² = 25%
20000' 6² = 36%
etc to
40000' 10² = 100%
and everyone (?) knows that TAS is twice IAS at 40000 ft.
If you're looking for a calculation to get TAS change after an altitude change - say ATC send you up 2000' - then take current TAS in miles per min and multiply by height change in 000s of feet. This is the increase in TAS you'll have got from changing altitude.