In B+ tree the order of root and internal nodes and order of leaves is different.
If we take case of leaves
N * (key + block pointer) + 1 block pointer linking to next leaf $\leq$ Disk block size (N = order of leaves)
N * (8+4) + 4 $\leq$ 512
12N $\leq$ 508
N = floor value (508/12)
N = 42
If we take case of root and internal nodes
N* size of block pointer + (N-1)* key $\leq$ Disk block size (N = order)
4N + (N-1) 8 $\leq$ 512
N= floor value (520/12)
N=43