Hi Anne,
Thank you for the question.
I would recommend soaking the cubes with 1.5 EC/ pH 5.5-6.5 from the start. Since you took the cutting off a plan that was receiving full nutrient and of the rockwool has none you want to supply the clone with some nutrients from the start. You do not have to soak for 30 minutes but just enough to fully saturate the blocks. Also depending on the dome and your environment I would vent the dome fully from the start. You are aiming for about 80% humidity. After you take the cuttings and place the dome I would wait about a week before watering again and transitioning the dome off over a day or two as your environment allows. Then you should wait a few days between the first few irrigations to allow the block to dry back some and the plant to develop roots.
Please let me know if you have any additional questions.
Best Regards,
Doug Jacobs
Grodan Technical Advisor