That is correct. Be careful with the formulas though because it is possible at low speeds that you will get no interrupts at all within 250mS so you could get strange results.
There is a 'trade off' between accuracy and how fast you need to see the results. Obviously to get the most accurate RPM you have to count over the whole minute. If you measure over shorter periods the answer will update more frequently but you lose accuracy, for example, if you use (count = count * 4) the result can only be in steps of 4. Personally, I would set the timer period so you are sure to capture at least one pulse at the slowest RPM, for example 5 RPM = 1 pulse every 12 seconds so use at least a 12 second interrupt period to count in.
Brian.