calc() {
    if (typeof this.value === 'function') {
      this.value = this.value();
    }

    if (typeof this.value === 'string') {
      this.calcString();
    } else {
      this.proxyValue();
    }

    if (this.isFinal()) {
      this.applyScale();
    }

    return this.outValue;
  }
Calculates value:
execute function, resolve var refs, convert string of (rem, percent) to pixels