fix: use max estimated airtime for duty cycle enforcement

Previously the max required airtime to enter the next transmit slot was
computed to be half the estimated time. However, the transmit logic uses
1.5 times the estimated time as maximum TX duration. By that, the duty
cycle could still be violated as the actual transmit logic might send
for longer than what we ensured to stay within the cycle.

We fix this by aligning both computations and always use 1.5 times the
estimated airtime.

Xref: #817

Author: fmoessbauer

src/Dispatcher.cpp

@@ -10,7 +10,6 @@ namespace mesh {
 
 #define MAX_RX_DELAY_MILLIS        32000  // 32 seconds
 #define MIN_TX_BUDGET_RESERVE_MS   100    // min budget (ms) required before allowing next TX
-#define MIN_TX_BUDGET_AIRTIME_DIV  2      // require at least 1/N of estimated airtime as budget before TX
 /// add some time to get the max allowed airtime for the est. airtime
 #define MAX_TX_AIRTIME_FOR_EST(est_airtime) (((est_airtime) * 3) / 2)
 
@@ -275,9 +274,9 @@ void Dispatcher::processRecvPacket(Packet* pkt) {
 
 void Dispatcher::ensureTxDutyCycle(int tx_len) {
   uint32_t est_airtime = _radio->getEstAirtimeFor(tx_len);
-  if (tx_budget_ms < est_airtime / MIN_TX_BUDGET_AIRTIME_DIV) {
+  if (tx_budget_ms < MAX_TX_AIRTIME_FOR_EST(est_airtime)) {
     float duty_cycle = 1.0f / (1.0f + getAirtimeBudgetFactor());
-    unsigned long needed = est_airtime / MIN_TX_BUDGET_AIRTIME_DIV - tx_budget_ms;
+    unsigned long needed = MAX_TX_AIRTIME_FOR_EST(est_airtime) - tx_budget_ms;
     next_tx_time = futureMillis((unsigned long)(needed / duty_cycle));
   }
 }