David I've not built a Skerry so perhaps more experienced hands will add their replies.

With my Waterlust project more than once I've taken Dillon's advice about "... 1/8" is close enough with stitch'n'glue..." while also striving to close what few gaps I've been confronted with (admittedly few) to my own satisfaction.

One of her mast pockets meets this criteria; having a solid bottom piece, the pre-assembled pocket needed some serious rasping to fit where the bottom meets the frame it's bonded to. The fillet along the bottom of that frame interfered when I dry-fitted the pocket so I made the pocket fit the fillet.

Early on it was clearly evident I needed to force the breasthooks into place along the sheer clamps to bring about the designed shape of the bow and stern sheers. This too came at Dillon's recommendation when I sent him a couple of pictures of what I was seening, before I started to finalize the shapes of those breasthooks.

There are some things that just don't get included in the manuals provided. Either the answers come from prior experience (you built a CLC kit before?) or you look to find them among more experienced hands such as those here, or through the CLC Builders' Club.

   In my building process, the same question came up.

My solution:  Place mast step on the floorboard, mark for and relieve (file or saw) it's forward lower corners to comply with the fillet arc. When back in position, this left the small gap at the lower vertical edges of the step. With a pencil held on it's side to the frame, scribe a line on the step's side - bottom to top. Repeat on other side of step.

Saw (or file away) along these lines to result in a tighter fit of the step's forward edges. Remember, as SPClark said, there's a bit of leeway in the stitch'n'glue building technique that can take care of minor gaps.

N6589G pretty much described how I approached the problem: recontour the machined-square bottom edge of mast pocket (step) assembly to allow closest fit to floor and frame it's going to be secured to.

The technique he describes (no pun intended, it's just the right word for it!) is how caninetmakers fit countertops to uneven walls - 'scribe-to-fit' - where no additional trim or backsplash will hide any gaps.

For my Waterlust the after-shaping fit was close enough I needed only handscrews (Jorgensen clamps, albeit a couple of BIG ones!) to make the mast pockets fit well 'enough' against the frames to my content.

Then Cell-o-fill thickened epoxy was slathered on pocket bottoms & mating edges atop a freshly-applied coat of unthickened epoxy before everything was permanently placed then clamped up.

(Uploaded image showing fitted pockets to Flickr but can't get URL to Preview image so I'm just gonna make that bold text above an active link to it if you want to see what I saw when I took the pic.... Let's see what happens now!)

(OK so that kinda works.

Despite proofreadings I missed that typo. Shouldabeen CABINETMAKER.

Sure do miss that <edit> button I know & love elsewhere.)