Estimating response of a filter given its order and roll off frequency
Unfortunately, knowing the -3dB and rate of rolloff above it may tell you nothing about the flatness of the filter below the -3dB point.

You need to know what type of filter you have. Three passband 'extremes' are the Bessel, the Butterworth, and the Chebychev response.

A Bessel filter is fairly predictable, as it gets higher order it gets closer to a Gaussian amplitude response.

A Butterworth filter is fairly predictable, as it gets higher order it gets closer to flat from DC to a substantial fraction of the passband.

With a Cheby, all bets are off, as the passband ripple is an important part of the design specification. Some applications may tolerate a 1dB ripple (or more) in exchange for a faster rolloff, some may specify 0.1dB ripple (or even less) for accuracy across the passband.