The answer is yes and no. It all comes down to how much of a perfectionist you are. There are a couple of ways to deal with this. But adding diluent to an existing concentration can get messy if you're not careful.
Here's the deal: You had 5,000mcg/1,000uL OR 5mg/mL initial concentration.
You administered 100mcg, which, at the original concentration, was 20uL (microliters) of your GHRP-6/BW solution.
If this is the only volume you used, then you now have 4,900mcg GHRP-6 and 980uL of BW left in the vial. This equates to a concentration of ~5mg/mL (the concentration doesn't change).
So
if the above is correct, you now have a volume <1mL. In fact, you'd have 980uL (or .980mL). The concentration is still 5mg/mL
Option 1: So now you want to dilute your peptide with another 1mL BW. Here's the issue:
4,900mcg/(0.980mL + 1mL) = 4,900mcg/1.980mL = 2,474.7mcg/mL = 2.47mg/mL. Now try working with that concentration...how on Earth will you be able to withdraw another 100mcg from this concentration? Maybe it doesn't matter.....the concentration in the vial is SO close to the original 5mg/mL that it won't matter. In fact, with your 1mL syringe, you'd have to draw to the 2.02 tickmark (4.04IUs). Does it really matter that you'd be off by .02 tickmark? Probably not. But this is
assuming the above scenario is correct (you only removed 20uL from the starting volume).
Option 2: If you really have a 4,900mcg/980uL solution and IF you add 980uL (0.980mL) additional BW to that concentration, you'd have 2.5mg/mL solution. You can work with that. Simply draw to the 2nd tick to get 100mcg GHRP-6.
If you want to keep life simple, don't add more BW. If it ain't broke, don't fix it. Could I have simply said this instead of go through that mess above? Yes, but I want people to understand, through numbers, what happens when you try to dilute a pre-existing fractional volume at a given concentration. It SEEMS like it'd work straightforward, but it can get tricky and may lead to messier habits.
Hope this helps.