When placing tacks into the styrofoam, you want to overlap them to both hold the previous tacks in place and to hide the underlying styrofoam.
When I place my tacks, I put the next tack in the very corner of the "V" formed by the previous tacks.
I saw you were still missing estimates for a number of the eggs. I plugged what you had into a spreadsheet (because I have a deep-seated need to do a spreadsheet for everything) and figured that if you multiply the height by the width by 55 and round up for safety, it'll give you a decent estimate.
Tutorials and images of DIY nail polish dragon eggs and related crafts.