Do I dilute ethylene glycol for use in a circulator? By how much?

Water-glycol can be used as a thermal fluid in open bath units and chillers (although it must not be used in Unistats).  Glycol is commonly used to reduce the freezing point of water, so you can work at low temperatures.

You do not use ethylene glycol neat, because it is so viscous.

You dilute it with water to reduce the viscosity and so improve the flow (and also make it more cost effective).

The exact proportion of water-glycol to use depends on what temperature you want to work at, as the proportion determines the freezing point of the mixture – and you want to make sure that the freezing point is below the temperature that the water-glycol mixture will be cooled down to in your system.  There are tables and graphs available online that show the percentage / freezing point ratio.

A mixture of about 2:1 water:glycol (around 33% glycol) is often recommended, as this has quite a low freezing point without being too viscous.

(Changing the proportion also affects the boiling point of the mixture.)