job_item.rb

class JobItem < ApplicationRecord

  VAT_RATE = 0.2
  AMENDMENT_FORM_LABELS = ['Lateness', 'Full Refund'].freeze
  AMENDMENT_TYPES = ['JobItems::Lateness', 'JobItems::Refund'].freeze
  CHARGABLE_TYPES = ['JobItems::GuardBooking', 'JobItems::LateBookingPenalty'].freeze
  REFUNDABLE_TYPES = ['JobItems::Lateness', 'JobItems::Refund'].freeze
  PERCENTAGE_TYPES = ['JobItems::CancelationPenalty', 'JobItems::LateBookingPenalty'].freeze
  HOURS_TYPES = ['JobItems::GuardBooking', 'JobItems::Lateness',].freeze

  belongs_to :job
  belongs_to :resource, polymorphic: true
  has_one :job_item_payment, dependent: :destroy
  has_one :payment, through: :job_item_payment, dependent: :destroy

  validate :quantity_is_present
  validate :guard_job_exists
  validate :max_hours
  validate :max_refund_exceeded

  before_save :set_vat

  def total_pay
    if quantity.present?
      if PERCENTAGE_TYPES.include?(type)
        (((pay_rate * self.job.number_of_hours) * quantity) / 100.0).round(2)
      else
        (pay_rate * quantity).round(2)
      end
    end
  end

  def total_charge
    if quantity.present?
      if PERCENTAGE_TYPES.include?(type)
        (((charge_rate * self.job.number_of_hours) * quantity) / 100.0).round(2)
      else
        (charge_rate * quantity).round(2)
      end
    end
  end

  def total
    (total_charge + vat).round(2)
  end

  def quantity_is_present
    errors[:hours] << "Please select a duration" unless quantity.present?
  end

  def guard_job_exists
    errors[:guard_id] << "Please select a guard" unless resource.present?
  end

  def max_hours
    return unless quantity.present?
    if HOURS_TYPES.include?(type)
      errors[:hours] << "The amount of hours can not exceed job's length#{self.job.number_of_hours}" if quantity > self.job.number_of_hours
    end
  end

  def max_refund_exceeded
    return unless quantity.present?
    amendments_amount = self.job.job_items.where(type: REFUNDABLE_TYPES).map(&:total).sum
    
    if amendments_amount > self.job.total_charge
      errors[:base] << "You've exceeded maximum refundable amount" 
    end
  end

  def vat 
    vat = (total_charge * VAT_RATE).round(2)
  end

  private

  def set_vat
    vat = (total_charge * VAT_RATE).round(2)
  end
end

Why do you have to work add column for type - can you not work out from Type of JobItem what quantity will be?
i.e.
if JI is lateness - we know it is hrs.
If JI is Penalty we know it is %

In Java we can do something like this

Switch (TYPE){ //Type of JobItem

case Penalty, LBP, CancellationCharge: {
//% do percentage calculation
}
break;
case GuardBooking, Lateness: {
//This is always hours
}
break:

} //Close Switch

With this approach we don't need new column of % or hrs.