"My usual dose is around 40mg maximum, but I have heard that CBD balances the THC out. Is this correct, or is this a really high dose of THC?"
70mg is a large single dose. For non-chronic users, approximately 2.5-5mg is a good starting dose for symptomatic relief and sleep. Daily users may require a higher dose. The product you mention is likely a multi-dose package and can be divided somewhat proportionally to estimate a smaller dose.
In general, the higher the CBD to THC ratio, the less impact of the THC on cognition. 30% CBD indicates that approximately 30 percent of the cannabinoids in the product are CBD. If the main two cannabinoids are THC and CBD in this product the ratio is 3 CBD molecules to 7 THC molecules (3:7). Some products contain additional cannabinoids in smaller amounts that may change this ratio slightly.