How to set a property to a proportion of a map effectively

I have a really big map on Netlogo and I want to tell it a a certain (changable by a slider) percentage of it has a coffee plantation. Howevere, the map is so big that its taking too long to process. The way i have it works but I´m hoping theres a faster way to do this.

I have

ask patches [set total-cultivation count patches with [Type-Cover= "Agricultural Areas"]]

ask n-of (total-cultivation * %area) patches [set plant-type coffee ]

Total Cultivation is a Global Variable
The %area goes from 0 to 1.

This is the map. The grey part is the one a need to count. Thanks in advance

 cobertura >= 18 and cobertura <= 18.99 or cobertura >= 19 and cobertura <= 19.99[set pcolor grey set Type-Cover "Agricultural Areas"]

Edit: I encountered another problem. Even if this worked patches would be placed at random, but plantations dont take just one pixel, they are mostly close together. How would i solve this?

You are asking every single patch to do the count. That means that N patches each evaluate the type cover of N patches, for a total of N^2 operations.

From how you are using it later on, I assume total-cultivation is a global parameter. So just cutting out the patches will already do the trick for reducing it to N operation.

set total-cultivation count patches with [Type-Cover= "Agricultural Areas"]

As for your second question, you will first need to figure out what the rules of your system are. How clustered are they supposed to be? You can find several earlier questions about clustering here on Stackoverflow, so I would suggest taking a look at those.