RT-PCR: Not seeing how it can measure mRNA expression levels
There might be a confusion of terminology. There is real-time quantitative PCR, which can sometimes be called RT-PCR. There is also reverse-transcriptase PCR, which is also confusingly called RT-PCR. To try to reduce confusion, real-time quantitative PCR is also called RT-qPCR or qPCR.
What you describe in your question is the reverse-transcriptase type of PCR. This is used for amplification, not quantification, of starting material.
QPCR or quantitative PCR, on the other hand, uses fluorescent dyes to measure the amount of amplified gene product. Standards provide a baseline measure of how much genetic material you should expect to see over time, at each PCR cycle, for a given amount of measured fluorescence. Measuring fluorescence of your own sample over time, and comparing it against the standard, gives you an estimate of what you started with, initially.