My husband is in the process of erecting a pergola in our garden, but I need
some advice

We have 4 upright posts - measuring 4" x 2" (height about 2 metres)

Have tried looking on the net to get an idea of the height, but can't find
anything.
I would like some help please with the following :

1. I don't want the pergola 2 metres high, but would like to know what would
be an ideal height?
2 Measurements required here :
a. what height above ground should posts be?
b. how much below ground?

2. Concreting posts into ground :
a. how many bags of sand and cement do we need ? the ratio ? should
the mixture be on the dry side ?

Can someone help with the above, as I need to get the sand and cement on
Friday afternoon for the weekend.