If holding .76 while descending. the TAS will not be constant due to the increasing TAS associated with the presumably increasing temperature as you descend. Another way to approach the problem might be to calculate the altitude at which 300 KIAS results a TAS which at that temperature is equal to .76 mach. I've sometimes seen or heard this is referred to as mach/IAS crossover altitude. I also seem to remember seeing a chart in an operational planning manual somewhere too. No matter.
Whenever I've gotten curious about crossover altitude I just estimate it with mental math. Close enough.
Using 2% per thousand feet as the approximate increase of TAS/IAS with altitude increase, track 300 KIAS to 25,000' to get about 450 KTAS. Now just assume the speed of sound to be about 600 kts at that altitude. 76% of 600 is just more than 450. Crossover will be near FL250.
Wanna check to see if this is the actual number? First get get out the calculator. 38.94 (or 39) x sq rt temperature in deg K at 25,000. calculate TAS of 300 KIAS at 25,000'. You could input the formulas into excel and make chart.
Or use your favorite search engine and the search terms "atmospheric properties calculator" to find a pretty handy online calculation tool.
Have fun!
westhawk