I'd like to start by saying ... that is a very good block! And bravo to you for doing a test block with scraps to work the kinks out!
Are you measuring your 1/4" seams AFTER ironing?
Are you using fabrics of different weights (ie - looks like you have muslin weight in there and the dark green might be a heavy weight fabric?). When I'm sewing small pieces (1/4" finished) or a block with a lot of pieces, I have to be very careful to either not use fabrics of different weights (ie I can't mix batiks and asians), OR adjust my seam allowance with every combination of fabric. The difference is that the heavier fabrics take up more space to fold over the seam than a batik or a muslin. The smaller the piece - or the more pieces you have (such as your block) can throw the whole block off.
You can test your 1/4" by pulling 3 of the fabrics you plan to use, cut each piece 1 or 2 inches (whichever is more consistent with the size you will be using) and sew them together. If you used 1" pieces your finished size will be 2.5" AFTER you iron it. If not, adjust.
And of course sewing over seam allowances needs to be compensated for as well.
Every time you have a seam you have an "opportunity" for the finished product to be off a smidge. The more seams you have, the larger the discrepancy in the end product. Each of these little smidges will multiple - naked to the eye until you look at the end result.
It's best to make sure that the parts within the block as perfect as possible - matching all seams and nice points. It's not at all uncommon that the outer edges of blocks are not perfect as a result (for all the above reasons) especially on a complicated block! I think the only blocks I can get to finish perfect on all edges is a square in a square! As long as you can trim the outer edges of the block and still have enough seam allowance to connect it to the next block (or sashing) you are OK.
Good luck!