[PyMOL] How to manually define the lower and upper limit of vacuum electrostatics?

Dear all,

I am trying to compare the vacuum electrostatics of three related proteins. I loaded them in pymol and use "Action>generate>vacuum electrostatics menu to generate the vacuum electrostatics of the three proteins. I find that the lower and upper limit of the gradient bars generated with the  vacuum electrostatics of the three proteins are different. They are -64.1~64.1, -72.2~72.2 and -82.2~82.2, respectively.  

I wonder whether I could define the lower and upper limit of the gradients to the same values so that the vacuum electrostatics of the three proteins can be better compared?

Best regards.
Yeping