Grid arithmetic
Three columns of 8 in frames with 2.5 in gaps measure 3 x 8 + 2 x 2.5 = 29 in across, and two rows of 10 in frames measure 2 x 10 + 2.5 = 22.5 in tall. Those two numbers are the entire outer boundary, so check them against the furniture below before marking anything.
The grid is the strictest layout
Identical frames mean every edge has a neighbour to be compared against, so levelling errors are visible at a glance. Mark all six hook positions from one horizontal datum line and one vertical datum line rather than measuring each frame from the previous one, which accumulates error across the row.
Salon blocks still need a boundary
A mixed six-piece block reads as intentional only when its outer edge is a recognisable rectangle. Keep the leftmost and rightmost frames aligned on their outer edges, and keep the top and bottom rows within a consistent band. Inside that boundary the sizes can vary freely.