"Is there any way to set an average bitrate?"

What you want to do with MegaPEG is set your ceiling, which is related to the bandwidth constraints of your mux (ie. what other tracks will be muxed) usually 7.5 mbs is a good ceiling.

Then, do some test encodes on the easiest sections of the material, and with each test encode successively lower the Size vs. Quality slider until you can see uniform artifacting in all the frames. This setting you can consider to be your 'minimum quant' or minimum compression, aka the 'quant floor'.

Back off the minimum compression (raise the slider) one or two notches.

Now, do a full pass. The average bitrate which will be shown by our graphs will be perfectly correlated with the encoding challenge/complexity of the material, which is exactly what the goal of VBR encoding is: perceptual constant quality.

This is an approach for program content which includes both challenging and
easy material. For very easy material, it is possible, with MegaPEG.X to acheive true constant quality. Normally the problem with true constant-Q is that the bitrate can spike outside the allowable channel constraints. However, with MegaPEG.X, we provide you with the exact encoding/diagnostic tools to be able to do a true constant-quality encode, and to verify that the compressed file meets the requirements for the limitations of your VBR channel.

The minimum quant, or quant floor will appear in your movie analysis: in the movie quantization graph, the quant floor will appear as the minimum that the quant drops to during encoding. This practice eliminates wasted bits, which is where the notion of VBR emerges. VBR means don't use bits if you don't have to.

When the quantization floor is set correctly, your picture quality will be perfect, but in the slow-moving scenes of your program material, the bitrate will drop down to almost nothing (in the movie bitrate graph, this is depicted as a valley, or low spot).