No prob. Just make sure the 10ml bottle you are adding to is as close to 10mls as possible. 100mg base has almost zero wiggle room with calcs.
If you have 10ml at 4mg and want 10ml at 16mg then why not just make another 10ml batch at 20 mg and combine the two which would give you 20ml at 16mg.
IMO evryone who DIYs ejuice really needs to learn how to use Ejuice me up or similiar calculator. As other posted 100mg Nic has to be handled with care and a posted typo could really cause a major mg error.