If you are looking to make sausage that you are going to hot smoke then the Sodium Nitrite will be fine however if you are going to make any cured meats that are going to be air dried then you will also need Potassium Nitrate. You mention sausage so will assume that you are going to hot smoke it.
The use of Nitrite in sausage is to protect the meat from bacterial growth in the short term whilst it is being cold/slow hot smoked until it is finally cooked. To use you just need to calculate the amount of cure to add to achieve a final level of Nitrite that is both safe to eat and is an effective antibacterial. You will need to take into account all of the ingredients that you are going to include in the sausage in order to calculate the amount of cure you need to add.
You are looking to achieve an end concentration of Nitrite of no more than 150 mg/Kg and no less than 50 mg/Kg. As you can see the effective range is quite broad. Ideally you should aim to achieve around 120-130 mg/Kg then you will be well within the acceptable range.
As CrazyMoon mentioned the cure that is usually sold is 6.25% Nitrite however to use your 5% you only need to adjust the calculation a little to take this into account.
An example calculation for a mix containing the following:
1,000 g meat )1Kg)
57 g Water (1/4 cup)
18 g Salt (~2%)
18 g Sugar (~2%)
10 g herbs
Total weight of ingredients = 1,103 g
mg/Kg required (Ppm) - 130
Nitrite in cure expressed as a decimal % - 0.05 (5%)
<amount of cure required> = (<mg/Kg required> * <total weight of ingredients>) / (<Nitrite% in cure> * 1,000,000)
In order to achieve 130 mg/Kg (or Ppm) Nitrite then for the ingredients above you need to replace 2.86 grams of the salt with your 5% cure.
Note - if you add 3g of cure you will get 140 mg/Kg Nitrite and if you only add 2g of cure then you will get 94 mg/Kg Nitrite - so anywhere in between 2 and 3 grams is fine. This is however a still very small amount of cure and this is why we recommend using digital scales that are accurate to 0.1 of a gram.