When casting onto circular needles how can I make certain that the tightness is correct?
Getting a circular cast-on up and running can indeed be tricky. Fortunately, there are several techniques that can help.

Use a sturdy cast-on The long-tail cast-on is a good one for this as it basically includes the first row of knitting. Be careful if you need stretch, though.

Add some weight to the cast-on This can be done by putting some stitch markers in the cast-on (helps get the number of stitches right, too!) or by knitting a short provisional section flat and then joining (you go back later, unravel the provisional section, and use that yarn to cast-off).

Try DPNs If the cable part is what grieves you, you might try casting on with double-point needles, instead. They can have their own fussiness, but it works for some folks.

Practice Yeah, nobody likes to hear this one, but it's so very true. Even after more than a decade of knitting, it still sometimes takes me a few tries to get a circular cast-on going well.

Additional resources:


TECHknitter's 3-in-1 circular join
Dee's No-Twist Circular Knitting Cast-On Tutorial