Since an expression syntax has been added to pgbench, I find that the readability of expressions is not great. An extra '=' improves the situation for me:
\set id = 1 + abs((:id * 1021) % (100000 * :scale))
seems slightly better than:
\set id 1 + abs((:id * 1021) % (100000 * :scale))
But that is debatable!
The attached patch just ignores a leading '=' in a pgbench expression.