This will give you a quick answer that comes faily close to the real deal:
Use 2% of IAS per 1000 ft and add to IAS:
Example: IAS is 200 Knots at 25 000 ft
therefore 2% X 25000 divide by 1000 = 25
= 50% of IAS
50% of 200Knots + 200 KNOTS = 100 + 200
= 300 Knots TAS