Use dynamic CSS based on the value of a field within a Lightning Web Component

In the general sense, it's best to incorporate calculated values into the data, so you can reference it directly:

<lightning-layout-item size="4" class={predictionRecord.Percentage_Style}>
  {predictionRecord.Percentage__c}%
</lightning-layout-item>

Which you then calculate as part of loading the record:

this.predictionRecord = {...this.predictionRecord, 
  Percentage_Style: this.calculateStyle(this.predictionRecord, 'Percentage__c')
};

This would call a function like:

calculateStyle(record, field) {
  return switch(true) {
    case record[field] >= 71: return 'high-percentage goals-minute-contents slds-text-align_center';
    case record[field] >= 50: return 'medium-percentage goals-minute-contents slds-text-align_center';
    case record[field] >= 40: return 'medium-low-percentage goals-minute-contents slds-text-align_center';
    case record[field] >= 0: return 'low-percentage goals-minute-contents slds-text-align_center';
  }
}

Also, as an advanced use case, you can use Proxy. I'm going to keep this relatively simple, so I suggest you read the documentation for more information.

Here, we're going to make a fake styling system using a proxy. Each time a field is referenced, we'll transparently return the value if it exists, otherwise we'll return a fake value we calculate in real time.

this.predictionRecord = new Proxy(
  this.predictionRecord, 
  { get:(target, key) {
      if(Object.prototype.hasOwnProperty.call(target, key)) {
        return target[key];
      }
      // from before
      return calculateStyle(target, key.substring(0, key.indexOf('__Style'));
    }
  }
); 

In this manner, you can then:

<lightning-layout-item class={predictionRecord.Percentage__c__Style}>

This is a way we can apply generic logic across many properties.

Edit:

The wire handler would be the place to change the data.

@wire(getStatistics, { recordId: "$recordId" }) wiredStatistics({ error, data }) {
  if(data) {
    this.predictionRecord = {...data, 
      Percentage_Style: this.calculateStyle(this.predictionRecord, 'Percentage__c')
    };
  }
}

Or, implemented as a proxy:

@track predictionRecord;
@wire(getStatistics, { recordId: "$recordId" }) wiredStatistics({ error, data }) {
    this.predictionRecord = new Proxy(data || {}, computedProperties);
    this.error = error;
}

To make this work, add the following code after the imports, but before the class:

function calculateStyle(record, field) {
  switch (true) {
    case record[field] >= 71:
      return "high-percentage goals-minute-contents slds-text-align_center";
    case record[field] >= 50:
      return "medium-percentage goals-minute-contents slds-text-align_center";
    case record[field] >= 40:
      return "medium-low-percentage goals-minute-contents slds-text-align_center";
    case record[field] >= 0:
      return "low-percentage goals-minute-contents slds-text-align_center";
  }
}

const computedProperties = {
  get(target, key) {
    if (Object.prototype.hasOwnProperty.call(target, key)) {
      return target[key];
    }
    const fieldEnd = key.indexOf("___Style");
    if (fieldEnd > -1) {
      return calculateStyle(target, key.substring(0, fieldEnd));
    }
    return null;
  },
};

To use the proxy, just reference the field with the suffix.

<lightning-layout-item class={predictionRecord.Percentage__c___Style}>