diff --git a/common/HostMemory.h b/common/HostMemory.h
index d016ddb58..07dae951b 100644
--- a/common/HostMemory.h
+++ b/common/HostMemory.h
@@ -22,7 +22,7 @@ size_t DefaultHostMemoryBudget();
 // The share of the budget that the large per-worker buffers of parallel steps - a translation function's
 // grids, a null replicate's model maps - may hold at once. The rest is what the run holds beside them:
 // its reflections, its merge, the detector-sized arrays.
-constexpr double HOST_WORKER_POOL_FRACTION = 1.0 / 3.0;
+constexpr double HOST_WORKER_POOL_FRACTION = 0.5;
 
 // One pool for those buffers, process-wide. A worker reserves its bytes before it makes them and hands
 // them back when it is done; one that would take the pool past its size waits until others have handed
diff --git a/image_analysis/structure_refinement/ModelGrid.h b/image_analysis/structure_refinement/ModelGrid.h
index 96f647292..b6075a999 100644
--- a/image_analysis/structure_refinement/ModelGrid.h
+++ b/image_analysis/structure_refinement/ModelGrid.h
@@ -36,3 +36,6 @@ size_t ModelGridPoints(const gemmi::UnitCell &cell, const gemmi::SpaceGroup *sg,
 // and the bulk-solvent mask grids, and for each of them its transform's input copy, spectrum and coefficients,
 // with the two made at once (ModelValidation's compute_model_factors).
 constexpr size_t MODEL_FACTORS_BYTES_PER_GRID_POINT = 32;
+// What a molecular-replacement placement's structure factors and rigid body hold on the CPU at their peak, per
+// point of that grid: measured at under half of the above.
+constexpr size_t MODEL_PLACEMENT_BYTES_PER_GRID_POINT = 16;
diff --git a/image_analysis/structure_refinement/MolecularReplacement.cpp b/image_analysis/structure_refinement/MolecularReplacement.cpp
index 53af8ec8c..5d976310f 100644
--- a/image_analysis/structure_refinement/MolecularReplacement.cpp
+++ b/image_analysis/structure_refinement/MolecularReplacement.cpp
@@ -2777,7 +2777,7 @@ MolecularReplacementResult search_model(const SearchModel &model, const std::vec
                 // gigabyte or more of grids on a large cell, so each reserves them against the host's worker
                 // pool first (HostWorkerPool), as a null replicate does.
                 const size_t placement_bytes = get_gpu_count() == 0
-                        ? MODEL_FACTORS_BYTES_PER_GRID_POINT * ModelGridPoints(cell, &sg, d_min)
+                        ? MODEL_PLACEMENT_BYTES_PER_GRID_POINT * ModelGridPoints(cell, &sg, d_min)
                         : 0;
                 for (First *f : to_refine)
                     running.push_back(std::async(std::launch::async, [&, f] {