How to transform field strength as a function of frequency
Here's what it says in the standard: -




  I don't know how to calculate the field strength for this frequency
  range as a function of the frequency


These are limits imposed by the standard - they are not related to frequency other than at the break point of 230 MHz.

So, for instance, at 300 MHz the actual volts per metre will be 223.87 uV/m.

This was calculated by raising 10 to the power 47/20 (\$10^{\frac{47}{20}})\$ i.e. converting from decibels back to real numbers.

Because the impedance of free space is (approximately 377 ohms) the power will be the field strength squared divided by 377 = 132.9 pico watts per metre squared.

This basically tells you that at 10 metres distant from some piece of electronics it must not produce a power density greater than 132.9 pW/m^2.

Given that a sphere of radius 10 m has a surface area of \$4\pi r^2\$ metres (1257 sq metres), the power produced at the point of generation is approximately 0.167 uW.


  But how can we transform this formula to include the frequency


You don't - it's not a formula - it's just a set of legal limits that happen to have a step change at 230 MHz.