Yes, the way you mixed it is fine (using the urine cup & also the 6.5mL water + zegerid + 10mL mylanta).
Like Tracy said, check out marci-kids for your dosing range. The way you mixed it above, your liquid will be 2mg/mL so you can figure out how much to give @ each dose (giving 3 doses per day).
Also (like the above post said) keep in mind that some babies can have very loose stools & tummy bloating from the Mylanta so if that happens you may need to move onto Tums Quik Paks (Tracy & I both had to for our babies).
Hope that helps!