Generally a load factor of 0.75 to 0.5 is recommended, which is where you might have got the 0.75 from.
But load factor = The no. of data entries / Size of Hash Table.
Which implies if you want a load factor of 0.75 you want a hash table of size 70,000 / 0.75 , or 93,000 - 94,000.
On the hash function for strings - a quick google I'm sure will give you examples.
Like this.