Further, if you are going to create the full seed yourself, then you need to calculate the checksum yourself, so you still need the 128 or 256 bits in binary to plug in to a SHA256 function. My method has always been to simply write down 0 for heads and 1 for tails (or vice versa), convert each 11 bits to decimal, and then map to the word list direct from GitHub (https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt). You don't risk the mistake of losing your place, you have everything written down so you can double check it, and you can easily use the number to calculate the checksum.