The beginning of the card is used for the partition table (1 sector for MBR and 34 sectors for GPT, where 1 sector=512 bytes). So with MiB alignment, 1MiB appears as free space (small part of that contains the very important partition table, so not really free). That means the size of your fat32 partition should be 19999MiB so that the ext4 starts at 20000MiB (and its size should then be exactly a multiple of 16MiB). When I first got the card, Samsung made the (only) fat32 partition start at 4MiB. So if that turns out to be optimal for fat32 (again, I don't know), then you'd make fat32 start at 4MiB with size 19996MiB so that the ext4 takes the rest of the space starting at 20000MiB. To be sure, you can use the command "fdisk -lu /dev/block/mmcblk1" in TWRP's terminal window. The partitions' starting positions will be shown in sectors and the starting sector of the partition you're aligning should be divisible by 32768 (16*1024*1024 bytes / 512 bytes per sector) for an erase block size of 16MiB. If you use a different card or later find out your erase block size is different, you can adjust accordingly. If I remember correctly, the size of the partition shown by fdisk is in what they call blocks, which are 1024 bytes each. So the number shown for the size of the partition would be divisible by 16384.
Anyway, this benchmarking business might not be accurate enough. SD cards don't support trim, so in order for the somewhat random erase cycles to not affect benchmark results, we'd probably need a larger sample size. Today I tried running the quick test a few times in a row, pausing a moment in between for the card to settle. The numbers aren't consistent enough to be really trustworthy when viewed individually. Most of the tries are about 30/15 or 31/16 with a bit of variability (just ballpark as I didn't bother to write the numbers down). But once in a few tries I'd get something like 32/10. And one time I got 28/18. Like I said, I can't say performance is really increased by further tweaking. I've also done some tweaks when I formatted the partition, but I don't know if those tweaks have had any effect. Still, partition alignment is the right thing to do. Even if it doesn't improve performance significantly (flash ram manufacturers are getting very good at making the cards perform well in many different kinds of applications), avoiding unnecessary block erasure should be good for the memory cells' lifespan.
Anyway, hopefully with more time spent using the card, you're happy with it. I like the boost in random write performance because it makes sense to improve the weakest link. When the better cards like Samsung's PRO or SanDisk's Extreme Pro/Plus come down in price, it'd be interesting to "upgrade" the phone's storage and see what performance faster cards get.