Congratulations on getting things assembled and running "Hello World’!
King Color Core per http://www.kingplastic.com/products/king-colorcore/ is a multi-layer “Starboard”. Starboard is HDPE w/ stiffeners, UV resistors, and some other chemicals added to make it less slippery than normal HDPE.
We have feed/speed rates for HDPE in the standard charts: http://docs.carbide3d.com/support/#tooling-support
and more at: https://www.shapeoko.com/wiki/index.php/Materials#Carbide_3D_Shapeoko_3_.28HDPE.29 — unfortunately, the feed / speed rates in Carbide Create are best for a Nomad — they will work after a fashion on a Shapeoko if one can turn the spindle speed down to match.
Some folks will use a calculator: https://www.shapeoko.com/wiki/index.php/Materials#Feed_and_Speed_Calculators (I’m still bummed that my favourite on-line one was taken off-line) and haven’t been able to bring myself to buy G-Wizard (since it wouldn’t be fair for me to do so, then to use it to populate all the data on the Materials page). Started working on understanding the underlying numbers, terminologies, and concepts here: Tutorial on feeds and speeds
The alternative is testing cuts — the Precise BIts folks have an excellent methodology here: http://www.precisebits.com/tutorials/calibrating_feeds_n_speeds.htm which I’ve used to good effect in the past.