Generally Bio-Spira comes in a packet that is enough to treat a 30g tank. For a ten gallon tank you would only need to use one third of the packet.
However, I have noticed from my experience and what I have heard from a few others that if you are already fishy cycling, it may take more than the usual dose and it may take longer to cycle the tank.
I *THINK* that is because in an already cycling tank there may be enough existing ammonia and/or nitrites that the bacteria have more than enough food to work with so it takes them some time to eat through it. So, you may end up using that whole packet in the end.
You really can't over dose (I guess you could if you have a tanker truck full of it but the packet won't do it
) so don't worry about it.
Tom