Another way you can do it is to take 500 feet per nm multiplied by your speed in nm per minute. 150kts is 2.5 nm per min.

500' per nm X 2.5nm per min = 1250' per min.

Don't forget that for larger climbs, your true airspeed will increase even though your calibrated/indicated speed remains constant.

TAS can be computed from Indicated Airspeed (IAS). TAS increases over IAS at the rate of 2 percent per 1,000 feet altitude increase. So, use the
equation:
TAS = IAS + (2% per 1,000 feet) (IAS)

Here's a nice resource available to anybody and I highly recommend it.

http://www.e-publishing.af.mil

There is a search function in the upper right. Type in "11-217" without the quotes. Make sure you have publications selected then click search. This returns two hits. Select 11-217V2 (V1 is all the boring stuff USAF folks must comply with), go to page 69 in V2. It's a great explanation of the 60-to-1 Rule and has a bunch of formulas.

Skyward80