I am filling a wide shadowbox with rocks and then resin to use as a tabletop. I will try my best to calculate the right amount of resin to use, but given the price of resin, I was hoping I could err on the lower side and then add later if I need to. For example I would plan for an inch deep but if I find that is not quite enough, add another quarter inch later. Is this a good practice? And if I do this, do I have to wait 48 hours before the first coat dries before adding the second? Is there any downside to this strategy?