use floating point arithmetic to compute the message size with overhead and use the size overhead in the computation of bid_per_byte

main.rb

@@ -139,11 +139,9 @@ post '/order' do
 
   order.message_size = message_size
   order.message_digest = sha256.to_s
-
-  message_size_with_overhead = message_size + FRAMING_OVERHEAD_PER_FRAGMENT * (message_size / FRAGMENT_SIZE).ceil
   
-  if bid.to_f / message_size_with_overhead.to_f < MIN_PER_BYTE_BID
-    bid_too_small_error(message_size_with_overhead * MIN_PER_BYTE_BID)
+  if bid.to_f / order.message_size_with_overhead < MIN_PER_BYTE_BID
+    bid_too_small_error(order.message_size_with_overhead * MIN_PER_BYTE_BID)
   end
 
   invoice = new_invoice(order, bid)

models/orders.rb

@@ -68,7 +68,7 @@ class Order < ActiveRecord::Base
 
   def adjust_bids
     self.bid = paid_invoices_total
-    self.bid_per_byte = (self.bid.to_f / self.message_size.to_f).round(2)
+    self.bid_per_byte = (self.bid.to_f / self.message_size_with_overhead).round(2)
     self.unpaid_bid = unpaid_invoices_total
   end
 
@@ -80,6 +80,10 @@ class Order < ActiveRecord::Base
     self.adjust_bids
     self.bid_per_byte >= MIN_PER_BYTE_BID
   end
+  
+  def message_size_with_overhead
+    self.message_size.to_f + FRAMING_OVERHEAD_PER_FRAGMENT * (self.message_size.to_f / FRAGMENT_SIZE).ceil    
+  end
 
   def paid_invoices_total
     self.invoices.where(status: :paid).pluck(:amount).reduce(:+) || 0