See the Wikipedia article on percentile

The 90-th percentile for your data is a value that is greater than 90% of the data. First sort your data into ascending order:

7, 12, 12, 23, 23, 34, 45, 45, 67, 99

which because the data is so sparse would strictly be 99. However Excel would calculate this to be 67+0.1*(99-67)=70.2.

Anything above 67 would be a 90-th percentile, but if it has to be a data value it has to be 99.

RonL