Do you just have a single tempdb data file? If you do, you should add at least one more, in which case you split the total size exactly evenly between them. For now, I'll assume just the one file and give you commands for that. If you want to use 2 files, let me know, and I'll give you those commands as well.
Note, too, that you are slowing processing down tremendously by forcing SQL to re-grow tempdb. Instead, tempdb should be set large enough that it never needs to grow (unless something truly unexpected happens). To do that, we need to change tempdb's default size, so that it starts out that big already:
ALTER DATABASE tempdb MODIFY FILE ( NAME = tempdev, SIZE = 16GB );
Depending on snapshot options, how much grouping/sorting is being done, how temp tables / temp variables are being used, etc., you could need a tempdb that actually is larger than the db itself.