diff --git a/src/acquisition.cpp b/src/acquisition.cpp
index fb006db..93640a5 100644
--- a/src/acquisition.cpp
+++ b/src/acquisition.cpp
@@ -143,32 +143,39 @@ std::uint32_t Acquisition::build_stamps(const udps_frame_t* frame, int time_sign
   stamps_.assign(n, 0.0);
   abs_stamps_.assign(n, 0.0);
 
-  // Relative mode: the stamp is a pure function of position and the *declared* rate. Nothing
-  // below this block runs -- not the arrival-cadence override, not the degeneracy guard, not
-  // the glitch bridge, not the monotonic clamp's reason for existing -- because a position
-  // cannot jitter, cannot fail to advance and cannot jump.
-  if (cfg_.relative_time) {
-    if (batch_ == 0) {
-      batch_ = n;
+  // Samples per packet, needed in both modes: relative stamping must advance `slot_base_`
+  // past the reserved hole before placing this frame's own stamps, and `on_data` must size
+  // the hole before calling this function -- both operations require knowing how many samples
+  // a missing packet represents, which is only knowable from the first frame that arrives.
+  if (batch_ == 0) {
+    batch_ = n;
+    if (cfg_.relative_time) {
       std::fprintf(stderr,
                    "[udpscope] relative time: batch=%u samples/packet, rate=%.6g S/s declared\n",
                    batch_, model_.partition().base_dt > 0.0 ? 1.0 / model_.partition().base_dt
                                                             : 0.0);
       std::fflush(stderr);
-    } else if (n != batch_) {
-      // A frame whose length disagrees with the first one is an inconsistency in the
-      // producer, not a loss: the counter still names one packet. Warn once and keep the
-      // established batch, which is only used to size holes -- the timeline itself is built
-      // from the lengths actually seen, so it stays exact for this frame either way.
-      if (!warned_batch_) {
-        warned_batch_ = true;
-        std::fprintf(stderr,
-                     "[udpscope] warning: packet carries %u samples but the stream's first "
-                     "carried %u; keeping %u as the packet length a lost packet is worth.\n",
-                     n, batch_, batch_);
-        std::fflush(stderr);
-      }
     }
+  } else if (n != batch_) {
+    // A frame whose length disagrees with the first one is an inconsistency in the
+    // producer, not a loss: the counter still names one packet. Warn once and keep the
+    // established batch, which is only used to size holes -- the timeline itself is built
+    // from the lengths actually seen, so it stays exact for this frame either way.
+    if (!warned_batch_) {
+      warned_batch_ = true;
+      std::fprintf(stderr,
+                   "[udpscope] warning: packet carries %u samples but the stream's first "
+                   "carried %u; keeping %u as the packet length a lost packet is worth.\n",
+                   n, batch_, batch_);
+      std::fflush(stderr);
+    }
+  }
+
+  // Relative mode: the stamp is a pure function of position and the *declared* rate. Nothing
+  // below this block runs -- not the arrival-cadence override, not the degeneracy guard, not
+  // the glitch bridge, not the monotonic clamp's reason for existing -- because a position
+  // cannot jitter, cannot fail to advance and cannot jump.
+  if (cfg_.relative_time) {
     double dt = model_.partition().base_dt;
     if (!(dt > 0.0)) dt = 1.0e-3;
     // The reserved hole for the missing packets advances `slot_base_` so this frame's