What would be the best practice to use lazy jacks with a rigid boom kicker?

The boom should be taking the weight of the sail. The LJ should only be bearing the weight and pressure of the sails in regards to keeping them contained in the boom. Your kicker should be taking the weight of the sail and boom. The LJ should not be bearing any of that lifting load. Now all that being said Just because you have a kicker or a rigid Vang. Doesn’t mean you don’t need a topping lift. Nice thing is that while you are not at the boat, a topping lift can take the strain off your kicker/rigid Vang and help it live a longer life. A topping lift can also double as a backup main halyard, or an extra way to get hoisted aloft if one of your other halyards fails. There is no one key with LJ. Too tight, there isn’t enough room on the boom for the sail, or a batten might get fouled. Too loose and the sail may fall out of the LJ.